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.