On your present rig you'd be better off with 1gb ram and spend the savings on the best video card you can buy with the leftover budget.
You'll need 2gb of new ram on your next PC so save the $ for then.
If you buy ram, you'll want to buy it in a configuration that will maintain dual channel ram functionality. So if you have 1x512mb stick, you should get a matching one for the second channel.
If you have an AGP slot instead of PCIe graphics slot on your mainboard, you need to buy an AGP card, this limits the card you could buy for it to 7600GT, 7800GS , or ATI equivalents.
I just went from 1gb to 2gb and I can't see any difference despite everyone else saying how great 2gb is

This was no real surprise to me, but I was hopeful for some noticeable improvement. I intend on keeping this old rig a while longer so the 2gb may come in handy when I start using more ram hungry apps.

Mainboard: Asus P5K-Premium, CPU=Intel E6850 @ x8x450fsb 3.6ghz, RAM: 4gb PC8500 Team Dark, Video: NV8800GT, HDD: 2x1Tb Samsung F3 RAID-0 + 1Tb F3, PSU: Antec 550 Basiq, OS: Win7x64, Display: 24&