Well, with a solid motherboard and by raising the BLCK to ~105 MHz (but this is something NOT RECOMMENDED) the Xeon can reach ~3.9GHz.
I just did a quick test and for check the stability, I had no time to play with the voltage etc... anyway 1.125~150V (but it all depends from your batch, luckily my CPU behaves very well in undervolting)
Generally they crash after 103~105MHz, overcome this limitation is more extreme and all depends on your motherboard & bios mods, if you have knowledge on the subject, ok at your own risk, but if you're not into overclocking & mods, it's better to not attempt this type of changes.
As I said, it all depends on many factors, for example you can crash and at a BLCK of 101MHz.