Before you go out and buy RAM I would check your motherboard documentation. Often times your RAM has to be paired up correctly for it to work right. You will notice the largest differance with an extra 256MB RAM. After that the differance gets expodentially smaller. I personally would go with 1 256MB stick or 1 512MB stick. Especially with the high prices of RAM right now. I have 1.28GB of RAM and I honeslty can't tell much of a differance from when I had 768MB. I did notice a large differance from 512MB to 768MB.
Brady is right,
I run 768 in my main machine because the difference was marginal compared to 1 gig
Check your documentation. the last thing you want to do is get the wrong type.