Depends on the games. Some games use 4 cores and will perform awesome. Some games will use one core but not need much power. Some games use so much CPU that they would perform better with a core i7 and a 4670 than with a phenom IIx3 and a 5970. Most games will perform decently with 3 cores. In most games your CPU will be good enough.
Even games such as Left for Dead 1 are lagging severely when considering the power of both my cpu and videocard. CT1615 you are correct, 2.3 not 2.1, my bad. is this due to having a slow clockspeed or something else?
Are you comfortable testing an OC? It will void any warranty on the chip, but my guess is you'll see moderate improvement if you can get it near 3ghz. If you do OC and it doesn't improve, then the CPU isn't the problem.