Each has a small diference in architecture and performance. In your list the I7 940 has the best performance overall. The I7 940 also allow on to use it in a multi gpu configuration if that is needed. In general, since you are using it for WoW, the i7 870 will do well enough. But if you want that extra power (for some reason) then get the i7 940. There are too many differences to list. But the best performing gaming I7 is the extreme version.
Visit here to learn the differences of each chip ->
http://www.intel.com/support/process.../cs-023143.htm
Drommon