Are you looking for a gaming system? Or just a general purpose computer? Have you set a budget yet?
Athlon II X3 450 @ $80 is a good choice, especially if you're able to get the 4th core unlocked and stable.
The success rate at getting the 4th unlocked is pretty good, but never a guarantee. It's more the 'luck of the draw'
getting a good CPU than the choice of motherboard.
Unlocking and overclocking; if you do get a 4th core unlocked you might find it's unstable when you try overclocking.
So you'd be left with a choice of overclocking with 3 cores, or staying near stock speeds with 4 cores.
To get a good stable overclock on 4 cores you'd have to get lucky twice. Once to get the core unlocked, a 2nd time lucky
for it to be stable overclocked.
+1 on WR2's post for a good starting budget gaming build except I would go with this mobo: http://www.newegg.com/Product/Product.aspx?Item=N82E168... has 4 slots for memory vs 2 on the A880G+ leaving room to toss in more in the future and its cheaper due to not having onboard graphics which you wouldn't need with a dedicated GPU.
P.S. I haven't had any luck unlocking a Athlon II X3 with the A880G+ I don't know if its the BIOS that came with the disk or I'm just extremely unlucky but I've gone 0/4. Can't get windows 7 64 bit to even try to load when trying to boot with 4th core active.
I did check the NewEgg customer comments to make sure people were having unlocking success with the A880G+.
It was the usual some did/some did not result you'll see on any ACC board it seems.
That A770E3 is a good choice, and less expensive too. Just update the BIOS before any unlock attempt.
I don't know if I ever updated the BIOS on the A880G+, I had a nightmare of an issue on the mobo of my 1st build ever. After I updated the BIOS on an Asus it stopped reading my core temps(I had tweaked a few things along with that BIOS update not sure what effected my temp readings) After that I haven't messed with BIOS updates unless I had too lol. I might have to update the BIOS on the couple of A880G+ I have sitting at home and retry unlocking those X3's now tho