Especially since you intend to overclock, I would recommend the Athlon IIx2 620 instead of either of those CPUs. I have mine at 3.25GHz on stock cooling with stock voltage hitting 50C on load, 25 idle. I must also say good choice on the motherboard.
Also, I would not think that you would be able to push the x3 much further in overclocking than the x4. Games are also starting to become more optimized for more cores. Right now notables are Dragon Age, GTA4 and FSX. I think in the long run you would be better with a quad core than a higher clock frequency. The x4 is definitely worth the extra $35.
If you really want one of those two and would not consider the 620, go with the 630. The extra core will be well worth it.
If you would like to take more of a risk and pay $35 less, then go for the x3 and try to unlock the 4th core
Good luck with whichever you choose.
Just an additional note: With gaming, the L3 cache of the Phenom IIs is worth a lot more than it is with non-gaming. Up to 10%. Not worth the extra cost for me though.