If you are running Windows XP 256, I think is the minimum you should have. Generally if you are doing anything real demanding you could get away with just getting another 256 stick. 512 is generally enough for most stuff. Going to 768 would pretty much garuntee that you don't run out of RAM (Although you may not even use 512).
My Desktop: <A HREF="http://Mr5oh.tripod.com/pc.html" target="_new">http://Mr5oh.tripod.com/pc.html</A>
Overclocking Results: <A HREF="http://Mr5oh.tripod.com/pc2.html" target="_new">http://Mr5oh.tripod.com/pc2.html</A>