Windows XP can recognise up to 2GB RAM, or 3GB if you make changes to the boot.ini file. You may be able to fit more in your computer but XP cant use it. Windows Vista can recognise 4GB RAM, but it reserves 1GB for system use, so it will still only show 3GB. No 32 bit OS can use more than 4GB, its a mathematical limitation.If you use a 64bit operation system, you will be be able to make use of 4GB.In my experience, XP works best with 2GB RAM, and VISTA is best with 4GB.You should get at least 800MHz RAM, faster if you can.4x512MB is faster than 2x1GB, which is faster than 1x2GB, but not noticably, so I'd just get whatever the bargains are. Ideally, for your XP System, I'd go for 2x1GB 1066MHz RAM if your board supports it, otherwise 2x1GB 800HMz.
Mitz from Tips4pc