Photoshop :: Advantages Of Multi Processor Systems?

I am considering getting a new system to use for my photo processing applications. I see that Adobe has releases LR v2.0 which supports 64 bit OS and has announced that CS4, when ever it is released will also support 64 bits, well at least for Windows. This being the case, switching to Vista 64 seems to be the way to go. BUT what about the processor? The choices seem to be a quad core version from either Intel or AMD or moving higher, a Intel Xeon processor. This gives me two more choices, use two 5100 series processors for a total of four cores or go up to two 5400 series processors for a total of eight cores. The 5100 series approach seems to use less power that the 5400's and can be clocked faster. Also, last year I had the opportunity to quickly run a bench mark with CS3 on a Dell dual Xeon workstation running Windows XP 32 bit OS. It appeared that the the system only managed to make use of only 4 of the 8 processors.

I have done a fair amount of searching and have not found any good answers to the question of which processor configuration will give the best performance. To keep this discussion reasonable I would like to limit it to one quad core processor vs. two dual core Xeons vs. two quad core Xeons.