The recent benchmarks here on Tom's show that even when running at 1080P, the HD 6770 is good for ultra high settings with 60+ FPS on Diablo 3. It isn't particularly CPU intensive either so the Pentium G620 would be more than fine.
Something like this should be good for ultra high settings @ 1080P:
Pentium G620
2x2GB G.Skill NS 1333Mhz
ASRock H77 Pro4-M
Seagate Barracuda 750GB 7200RPM
ASUS HD 7750 1GB
SeaSonic S12II 380B 380W
Fractal Design Core 1000
LG DVD Burner
Total - $486.92