I'm running my 600 at 900MHz stable with an Alpha PAL6035 w/4500RPM fan. I tried a FOP38 (6800RPM fan), but as Krish mentioned, it's LOUD! Also, I think I needed to smooth the surface some, as I couldn't run stable at 900MHz anymore with the FOP38. I'm back to using my Alpha for now...
FOP38 (or maybe FOP32) is a bit cheaper than the Alphas, so something to consider if your short on cash and don't mind the noise.