120mm length? You sure? The majority of power supplies people buy at retail these days have a 120mm or larger fan—if that's the scenario, then it has to be longer than 120mm. In fact, the original ATX standard length is 140mm. Maybe you're talking about 140mm? The Corsair AX series 850W and below, and corresponding Seasonic X Series, are 160mm.
Silverstone recently released new Strider Gold power supplies at (550W and 650W) that happen to be fully modular and 140mm deep. That's enough power, except getting close for the 550W for a max overclock on that system, running prime95 + FurMark or similar only. Silverstone power supplies are usually pretty quiet; they have those large fingered heatsinks after all.
There's a SFX form factor 450W variant with an ATX mounting bracket; that one's actually under 120mm and could still handle the system, but not on any heavier overclocks. It probably wouldn't be quiet for those kinds of loads either.