Difference between memory of, say, 512MB DDR2 SDRAM 2 DIMMS and 512MB DDR2 SDRAM 1 DIMMS? And, SoDIMM?
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The differences between these memory chips are speed, types, and compatibility.  In addition to the DDR (double data rate), there is also speed, usually follow by DDR3200 or DDR4200.  The higher the number, the higher the speed.  You should check what is the maximum the memory the laptop can take. Most laptops only allow memory upgrade up to 2 Gigs.
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I don't know where you could find a list, but the 200 pin sodimm has only been common for a couple years.  It's a pretty safe bet that anything older than 3 years old can't use that RAM.  It is also a safe bet that anything less that 2 years old can use that RAM.\n\nI would try a RAM manufacturers website like Cruical.com or the like to see if they provide some sort of memory selector.  That might help
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2 x 184-pin DDR DIMM sockets\n\nSupport up to maximum 2 GB memory (PC manufacturer's maximum memory may differ)\n\nSupports unbuffered PC2700/2100 non-ECC DDR SDRAM memory
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1GB single DIMM\n4 - 16 GB multiiple DIMMs
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ddr and sd ram are both synchronous -- meaning that\nrather than responding as soon as an input signal\nchanges, they respond based on a clock cycle.  sd ram\ncan respond once per clock, whereas ddr (double data rate)\ncan transmit data on both the rising and falling edge of\nthe clock.\n\nddr2 is the same design as ddr (synchronous, both edges),\nbut it has been modified to run at a higher clock rate.\nFor large transfers, it can simply transmit more data\nin a fixed amount of time.\n\nYour computer was designed for some fixed types of memory.\nIf it was designed for a synchronous interface, then\nyou had better use synchronous memory.  Similarly, if\nit was only designed for only ddr memory, then it's not \nlikely to work with ddr2.\n\nAs for mixing *sizes* of memory, let your machine specs\nbe your guide :-)  I have worked on servers which required\nmemory to be installed in banks of 2 or 4 slots at a time\n(and each of those had to be identical).  On the other\nhand, I have a 256MB and a 512MB in an old laptop, and\nthat works fine.  My point is that where ever you buy \nyour memory should be able to look up what is supported\non your particular machine.
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The technical answer... DIMM = Dual Inline Memory Module.  DDR = Double Data Rate.  Really, they are independant of each other.  To really confuse you, DDR (and DDR2 for that matter) memory comes on a DIMM.  The DIMM can have different configurations, and as mentioned already... different speeds, different timings, etc etc.  So, the answer is not a simple one.  The best thing to do, as mentioned already, is to check your motherboard manual to figure out what kind of memory will work.  If you bought a "manufactured" PC from a big company, this may not be in your manual.  You may have to go to a memory manufacture's website to figure it out.  See my link in the Sources section below.\n\nThanks,\n\nGraham.
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Some of these answers are wrong and others are not clear enough.\n\nYou need to first check with either the PC maker or motherboard manufacturer and see:\n\n1. What is the total amount of memory supported by the board?\n2. Does this memory have to be installed in pairs?\n3. What type of memory am I currenty running so I don't break Double DDR by installing a single DIMM\n\nBasically your usually better off with multiple chips rather than a single large chip. Your cost per MB is usually lower this way plus if the system is designed to use multiple channels you get added bandwidth by splitting the memory into seperate banks on seperate busses.\n\nCrucial has a pretty good web site for finding out what memory your system can take but don't take that for gospel.
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2GB is the most memory your machine can handle
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I would recommend going to http://www.crucial.com/index.asp and using their memory finder if you ever have another question like this.  They also have an auto detect memory feature (see sources) if you don't know the model number or it's a custom built computer.  Once you know, then buy it cheaper somewhere else like newegg.  Give a man a fish and all that...
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SODIMM is made (mostly) specifically for laptops.  SDRAM is a slightly older type of memory (PC100, PC133 anyone? :).\n\nIf you're looking to upgrade a notebook, search by the model number to find out exactly what type of ram, maximum speed, size, and slots it can accomodate.
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DDR2 is faster DDR.  It's the natural evolution of the DDR SDRAM technology.  It's bolts on to the DDR technology some new requirements, such as \n\n"Off-Chip Driver calibration (OCD) and On-Die Termination (ODT). DDR2 also incorporates a larger 4-bit prefetch, additive latency, and enhanced registers."\n\nhttp://www.corsairmemory.com/corsair/products/specs/ddr2_faq.pdf#search='DDR2'\n\nThese additions enable the memory to be clocked higher due to a better signal integrity.  Although the rated speed of 667 Mhz and beyond make it seem like it's heads and shoulders about the current technology, real world benchmarks show that the DDR2 technology provides only a slight advantage over the older DDR technology.  And the increased clock speed also comes at the price of increased latency.\n\nHere is a benchmark done on a same system that supports both DDR and DDR2 memory.  The only caveat is that the DDR2 memory is rated at 533 Mhz which is the same speed as the DDR memory.\n\nhttp://www.overclockercafe.com/Articles/DDR_vs_DDR2/pg_3.htm\n\nHere is another benchmark done by another website, showing the marginal difference in performance provided by DDR2.  Pay special attention to the effect of the latency setting on the overall performance.\n\nhttp://www.xbitlabs.com/articles/memory/display/ddr2-ddr_5.html\n\nUntil DDR2 matures and can provide decent latency while providing higher clock speed, you will not see a huge improvement over the older DDR technology.
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Static RAM and Dynamic RAM
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dont use ur old stuff unless its ddr2 ... if its older than 6 months it isnt ... dont mix the types or you'll slow everything down , just add another 512 if you have a SINGLE stick, and check out the site the guy above mentioned to get the same speed memory ... if you have two sticks of mem now they are 256 each and most likely in dual channel configuration ... in that case i would get the same speed memory but buy (2) 512s' or (2)1gig sticks ... if you break up a pair of sticks in dual channel or add a third you will slow things down also ...
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DDR(Double Data Rate)ram. or sometimes called PC 2100/2700/3200\nBoards a couple years old used SDRAM or Rambus Ram
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DIMM's are dual in-line memory module. SIMM's are single in-line memory modules.\n\nThe main difference between SIMMs and DIMMs is that SIMMs have a 32-bit data path, while DIMMs have a 64-bit data path. Since Intel's Pentium has a 64-bit bus width, it required SIMMs installed in matched pairs in order to use them. The processor would then access the two SIMMs simultaneously. DIMMs were introduced to eliminate this inefficiency. Another difference is that DIMMs have separate electrical contacts on each side of the module, while the contacts on SIMMs on both sides are redundant.\n\nRIMM's are Direct Rambus DRAM or DRDRAM (sometimes just called Rambus DRAM or RDRAM) is a type of synchronous dynamic RAM, created by the Rambus Corporation. They're the same thing as DIMM's, except they're faster, and have built in error checking. They were very expensive at first, and no major computer manufacturer uses RIMM's.
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Yes you can.. I have 512 and 1024 in mine - as long as the memory is the same type and if they are different speeds - your PC will run both at the slowest of the two's speed as long as they have SPD (Serial Presence Detect) which autoconfigures your SODIMMs.
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http://computer.howstuffworks.com/question175.htm\nhttp://computer.howstuffworks.com/ram.htm