pranaearth, although the 8790 is supposed to be able to take future Prescotts, there's no qaurantee it will. Barring a pin change, the mostly likely problems will be as noted above, power and cooling.
Frankly, I believe that within two years the 8790 will be pretty much obsolete anyway. There is the coming PCI Express, BTX, memory changes, drive changes, display changes and the probable switch to 64-bit. All of these will be mainstream within a year, two at the most.
The 8790 is a formidable machine, but its likely to be the last of the PCI paradigm. Sort of like with the switch from battleships and cruisers to air craft carriers and submarines as the primary naval attack systems or switching from prop to jet.