I'd probably keep OS X on mine, but I can imagine simultaneously running every major x86 OS inside VMware Fusion virtual machines. That or a lab's worth of Vista VMs, plus a Server2008 or two.

And to think of how many NT4 VMs could potentially run comfortably with that much power.

And heck, it'd make a great photo processing box too -- but such power is out of the question for a few years yet.