RAM. Lots of RAM.
There's no difference between the i7 series and the Xeon 5500 series - same chip, only the Xeons are better binned for long-term use and have a second QPI to work in multi-CPU systems.
Same as the six-core i7 and the Xeon X5600 are the same.
So, if you want a dual-CPU system then you can ONLY use Xeons, otherwise it's entirely up to you, but you really can't beat the i7 950 right now.
And again, LOTS of RAM - Patriot have just released a 24GB Patriot Sector 7 kit, and most manufacturers are ramping up 12GB kits now that 4GB modules are viable.