Tony mate, run the memory scanner here:
http://www.crucial.com/uk/systemscanner/index.aspxThis will tell you what you've got and what upgrades are available for RAM. Bare in mind that if you are running 32bit XP and you install 4GB or more RAM, XP will only see just over 3.5GB.
I upgraded my VGA from an old 6600GTX to a GTX460 and games like CSS have gone from about 45fps to well over 200fps maxed out but newer games haven't had much of an improvement because my old CPU and mobo aren't quick enough for the graphics card lol.
I run 3GB RAM on XP and that's more than enough for the time being.