256 might be all you need, it really depends on what you want to do with you PC. I increased all PCs to 512MB well those with a limit. To me 512MB isn't enough 1GB of DDRAM is good start. 2GB and 4GB for those who really need power. Average users can get by with 512MB for desktop and laptops. It's when you get into video and audio editing then you need 1GB of DDRAM. If you want to get into HTPC (Home theater Personal Computer) then you need at least 1GB to capture and watch TV on your PC, just like turning your PC into DVR (digital video recorder).
So to answer your question get the extra RAM you PC will thank you, but one thing before I go you need to change the page file.
512x256 = 768 min
768x768 = 1536 max
If you don't do this? Adding the additional RAM won't help you much. This has to be adjusted.