Kestrel_909,
The problem with Dell, Compaq, HP, Gateway, etc. is they are dead end computers. Usually you cannot upgrade the processor, since the CPU they use is usually the highest speed that motherboard can take. Also they use non-standard parts so you have to go back to them to upgrade, and they charge so much to upgrade they usually talk you into buying a whole new computer from them. (That is the rule, but there are exceptions.)
For your computer, I would suggest getting your total RAM up to 512Mb. (If you are running WinXP.) There is no need to go higher in your case.
As for graphics, the GeForceFX 5700 Ultra and the ATI All in Wonder 9600Pro are currently best bang for the buck.
Making those two upgrades will give your framerates the much needed boost.
Take Care,
Richard