First, you must tell us what your motherboard, cpu and RAM (DDR or DDR2 or DDR3) are.
Second, tell us how much you can afford to spend?
Third, what is your monitor resolution & current video card.
Only after knowing this we can make the recommendation that is best suited for your need.
Assuming your motherboard is a step up from that you should be able to run the C2Q 45nm CPUs. If so you should be able to run a Q8300. <--- link
This is only a $150 upgrade and you'll have roughly the same (or more) performance of an Athlon II X4. OR if you live near a Microcenter you can find some awesome deals on C2Q CPUs (like a Q9550 for $190).
Q8300+2GB more ram is what I'd do. But if you're looking for an entirely new setup, an Athlon II X3 + cheap mobo + 4GB DDR3 1333mhz+ would be good.
Look into upgrading your GPU (to a 5770 or better) if you really want substantially better gaming performance.