DDR is a form of SDRAM. I'm not sure whether this will help but the definition of DDR SDRAM can be seen by click here and the definition of SDRAM by click here . The amount is RAM you need depends on what you use the computer for. If high quality graphics are needed then the more the better (within reason), but for a basic setup using office applications will not require alot. Crucial are good for buying memory and there is a manufacturer search that will help you find the right RAM for your machine. click here .
You current specification of computer will also have some bearing on what you should do. If the processor is old and relatively slow then there is little point in filling the system with RAM as it will not be put to good use.
Where did you buy the system from ? Do you have the manual for the motherboard, as that will tell you the type of RAM that you need. Then you can use the Crucial site to browse for the memory type you need.
To find out what memory you have in your computer, take one of the memory cards out and look at it. SDRAM is 168pin and has two cut outs on the bottom edge (offset so you can't put it in the wrong way) DDR memory is 184pin but with only one cut out (again offset so you can't put it in the wrong way). Both these memory cards are inserted upright and pushed down firmly. If your computer is fairly old you may have 72pin SIMM memory (difficult to find now) but the memory slots are usually White. SIMM memory can only be fitted in pairs.
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