I can't seem to find the M2 on the ECS website. I did however find the M and the M3.G31T-M
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
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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.