I just built a very similar system for a friend, if you have any questions about how it runs let me know and I'll see if I can get you answers (benchmarks, frame rates, etc). I know she's been playing GTA IV and Fable 3 on 720p on it, but I'm not sure what the framerates and settings are at.
I built a
Silverstone GD04
Silverstone 400W PSU
Gigabyte H61-M
Celeron G540 2.5GHz
stock intel cooler
4GB Corsair XMS 1333MHz
EVGA GT 630
Asus N15 PCI WiFi
Hauppauge TV Tuner card
500GB WD Blue
I didn't test it but I can tell you the GT630 seems weaker than say an HD6670 and it's not much cheaper, but I've used HD6670 on two other Celeron/Pentium systems and had problems on both with BSOD on loading Windows with the card installed. Those Celerons/Pentiums run extremely low temps with the stock cooler which is nice. I would like to see benchmarks comparing the two however, as I don't think their is much to gain from the Pentiums over the Celerons except for a few hundred MHz clock speed and a slightly larger cache. The GT630 just seems too weak to really consider, but unfortunately the GT640 is almost twice the price. NVidia really needs to drop the price on the GTX 650 and lower cards.