Upgrade to about 3-4gb of ram.
Pick up a 260 or 4870 and you should be fine as long as that has pci-e x16 which it should.
For higher demanding games you can't buy that cheap of parts and expect good performance.
Current Rig-I call him Q Core for short
Intel(R) Core(TM)2 Quad CPU Q8200 @ 2.33GHz 2.34 GHz|6.0GB RAM|NVIDIA GeForce 9500 GT 1GB|1.5TB @ 7200 RPM|Sound Blaster X-Fi-Xtreme Audio|Vista Home Premium
'"Crapbox 2.0.....Until you put Mac OS X on it, then its called a Mac Pro." -cavemangrinder
"You were sick, but now you're well again, and there's work to do." - Kurt Vonnegut