Can any i7 be used in pairs, or only some kind of Xeon variant with a different socket? I'm not sure at all, but this can become expensive.
Also, check which OS licence accepts two sockets. Can get expensive as well.
As well, if each socket has its own Ram, you need the OS variant that is aware so it puts the right data at the right place.
Keep in mind that Intel is to offer a six-core socket 1366 at the beginning of 2010. This may save you troubles for a desktop use.
Vista/Win7 Professional/Ent/Ult let you use 2 socket systems. I think you would need a Xeon, but don't quote me on that.
Like that guy said, 6 core will be nice since it's 50% more cores AND a die shink.
I'd rather have one 6-core i7 with 12MB of L3 cache than dual socket 4-core i7 with 8MB cache each. The dual socket will definitely make it cheaper to add lots of memory though. 6 ram slots per socket filled with cheap 2GB sticks for cheap 24GB of ram.