Big difference between DDR and Non-DDR ram?

[stso]

Does it really matter???
If I have a motherboard that supports DDR ram, do I "must" use DDR memory??
Is system with DDR much faster than non-DDR system?

Edit: Planning to get IWILL KK266 motherboard with Duron 600mhz processor. So I think my best choice is to buy the non-ddr version since there are not too much difference between them. Also, I can save some money on the RAM I already have ...

 

[esung]

It depends on the DDR MB design. DDR SDRAM and regular SDRAM use physically different slots. DDR has 184 pins and SDR has 168 pins
Some MB will have both type of memory slots you can choose one(presumebly that you want to use exisiting SDRAM first and upgrade later). but some MBs will only have DDR slots (to save cost I guess).
In this case you have no choice but using DDR.

Although the memory bandwidth of DDR is double what SDR have, not all program will benefit from the increased bandwidth, part of it is because it's fairly new, and program/compiler hasn't been optimized for it, and the other part is maybe the chipset itself is not mature yet. but I'd say you'll still get a a 10% performance increase, due to that fact that memory bandwidth will no longer be one of the bottleneck (at least compare to regular SDRAM)