300 bucks should get you a Core2Duo E7500 CPU($120) , Radeon 4850($100), and a power supply upgrade nice 600Watt ($80).
The cheep way to go would be to see how far you can overclock (the E2180 is known to have a decent overclock) the cpu, install a radeon 4670 which is good with low end power supply's($70), and upgrade the ram to 4GB($20) guessing that you have a fairly low res monitor those three steps would save you a few hundred bucks and let you play any game out right now. HAVE FUN!!!
I used to play WoW, had no problem playing that (at pretty low settings though, with vista). I am going to be playing Aion when it comes out as well as newer FPS games. I want to run Aion at full settings but the newer games that come out I just expect to be playable.