DDR-SDRAM

Well if u have a pentium 4 then it can use SD-RAM as well as DDR-RAM.
It all depends on ur motherboard.
The best thing is to check out ur motherboard manufacturer and get
details from their website for the memory. Or click on
www.motherboards.com/ and click a link to ur manufacturer.
DDR-RAM is faster than normal SD-RAM.
And the fastest is the RAM-BUS which is only supported by high end
motherboards.
Check out the motherboard manual for RAM support and what is the FSB
for ur motherboard. (100, 133, 200, 233 and so on)
SD-RAM is available in 66, 100, 133 and so on...
DDR memory is available in 266, 333, 400 Mhz and so on.