One nice feature of the Core i7 is a dynamic clocking process. If it detects a core or two being pegged by a process, but other cores idle, it will turn up the clock rate a bit to speed up the cores in use, while shutting down the idle cores.

I'm not sure how well this works, but it's an interesting feature.

As for adding an extra processor later to the Mac Pro, odds are it will cost quite a bit more after the fact then just buying the dual CUP machine day 1. The cost a friend of mine was quoted for the Early 2008 Mac Pro CPU at 2.8GHZ plus Apple heatsink was $1100. The option to go from quad to dual quad was only $500 when the machine was being ordered.