If I were you I'd spend an extra $15 for this Newegg.com - ASRock 880GM-LE AM3 AMD 880G Micro ATX AMD Motherboard
And then skip the AMD Athlon II x4 and go for a Phenom II x2 550 Newegg.com - AMD Phenom II X2 550 Black Edition Callisto 3.1GHz 2 x 512KB L2 Cache 6MB L3 Cache Socket AM3 80W Dual-Core Desktop Processor HDZ550WFK2DGI - Processors - Desktops
OR a phenom II x3 720 Newegg.com - AMD Phenom II X3 720 Black Edition Heka 2.8GHz 3 x 512KB L2 Cache 6MB L3 Cache Socket AM3 95W Triple-Core Processor HDZ720WFK3DGI - Processors - Desktops
The reason I mention this motherboard is that it's crossfire ready, has the new 880 chipset (which is nice) and is able to unlock extra cores. The Phenom II x2 and Phenom II x3 chips are all actually quad core chips. Your mileage may vary, but, I have had 2 of the Phenom II x2 550s and both worked fine when I unlocked the 2 extra cores (that's a quad core chip for $80) I also use the motherboard I linked.
FYI my Phenom II x2 550s run 4 cores @ Phenom II B50 (no idea where that model come from, that's what CPU-Z calls it) 100% stable at 3.7ghz.
I see that someone gave the motherboard a bad review, but I don't know what his deal was. I didn't have any issues with mine at all.
Like I said your mileage may vary but your chances are good for unlocking the cores. And if for some reason you don't unlock the cores, that Phenom II is a black edition which is easier to overclock, and you'll still be running any game you throw at it with impressive framerate.
I also recommend the Radeon 5770 over the 5750. It's only $10 or so more expensive, and the performance difference is well worth it.