I am now running my P5E3 Pro X48 + Xeon X3363 at 400 FSB with stock vcore (1.1625V, seems pretty good). This is only 3.4 GHz but it is very stable.
My gigabyte P35 was horribly unstable at 400 and higher, haven't even tried under 400, I think there is something wrong with it.
445 FSB is almost stable, did get a BSOD after an hour of gaming, and IBT on very high does give an error after a while. I used vcore 1.35V and it did get quite toasty around 75 C (hyper 212 EVO). 450 FSB is rather unstable. Anything higher is pretty much impossible. Highest boot is 495 FSB, but weirdly this only works sometimes? I set 495 FSB in BIOS, it switches off, no POST, press reset button, it POSTs (and then crashes first second loading windows).
Edited by 4everAnoob - 5/3/15 at 2:42am