Wow xeon E5 2600 @ 4GHz I guess dat would make PPBM5 dance, http://www.boxxtech.com/Products/3dboxx-8980-xtreme
Maybe so. But all current Xeons are multiplier-locked, so the only way to overclock them would be to raise the BCLK. However, most boards that support the E5 Xeons also support the 125MHz BCLK strap setting, so one can achieve around 4GHz without raising the other buses out of spec. However, not all octo-core Xeons can run stably at even 3.5GHz, let alone 4GHz. In fact, you might get a dud CPU that won't run stably at anything above stock speed.