mmm..ok i will tell u the most important thing about why DDR and DDR2 ram matter a lot.
DDR 400mhz rams can cost $100+/- for 1 gb while DDR2 533mhz/667mhz rams can cost $55 +/- for 1gb.. the price is nearly double.
for the price u spend to build a 2gb DDR machine, u can built a 4gb rig machine...
In a sense, why not just upgrade ur whole computer now, IF there is really a very big need....
Sidenote: i am having tt same headache as u now, mine pc is the small form factor type, really small one, and it cant fit the bigger cards. second, my power supply is only 250watts, and many of the newer cards require >350 watts.thirdly, my comp can only support AGP cards. Fourth, mine comp is only 2.26ghz and using DDR rams, its really not economical to upgrade the system at all. I m thinking of just rebuilding a new comp from scratch up again..maybe u can consider this too?