Okay, I have an Opteron 148 overclocked from 2.2 GHz to 2.8 GHz. That's the same thing as an FX-57, except I have a different core and FSB. But really, it's the same thing. Same speed. Running at 1.425 volts, up from stock at 1.4 volts. Ran fine at 1.4 but Prime95 quit after 6 hours, so just to be safe I bumped it up to 1.425. Top temperature is 51-52 Celsius which is fine for an AMD. They are STABLE! You won't regret getting one. I have an AMD chip clocked at FX-57 speeds and I saved 600 dollars. Let's be sensible: it's a good choice.
By the way, Xeon's are bad for games. Their architecture (I believe that is the determining factor... not completely sure) is awful for games. If you game at all, Opteron > Xeon by a margin you won't want to pass up.