The newer K10 processors are better than the old Phenoms. The Athlon II 620 is more powerful than any of them. DDR3 is a bonus as well.
For the motherboard, definitely grab an AMD chipset. Nvidia doesn't allow xfire and has no ACC.
CPU- Athlon x4 630
Motherboard- MSI 785G
RAM- Corsair 2x2GB
PSU- OCZ StealthXTreme 600W
That should give you a good base to work from. The motherboard could be a bit better, but it'll do what you need, overclocking included. This one would
be a better choice, more expensive though it may be:
If you ever plan to go crossfire, though, definitely spring for a 790FX chipset. Pricey, but they have x16/x16 crossfire/sli.
As far as the GPU goes, from what I understand any 5xxx lower than a 5850 isn't worth it, because they won't be powerful enough to run DX11 games properly anyway.
Case- NZXT Beta EVO (best value on the market IMO)
Hard Drive- Samsung Spinpoint F3 (some of the fastest drives available...cheap too)