The C2D at 3GHz should be near the top of its class so I would not bother with changing it. I might look at the video card instead.
I have an Intel Core2 Duo 3.00Ghz CPU and have been advised to upgrade to a better CPU as the one I have is going to be stopped from being made anymore. Of course it all depends on what other Intel CPU will be compatible with my MoBo which is an ASUS P5KPL-CM. I'm pleased with what I have and don't want to upgrade any other components if I can help it as I did a major upgrade about 18 months ago and the PC is perfectly adequate for my gaming needs. Please advise!