You're sure about i7? While I can't speak personally, for the performance you're looking for, the Phenom II's seem like a viable alternative as well. With the X58 motherboards breaking $300 even for the midrange ones, and the i7 920 running anywhere between $300-400,
this sort of thing might be worth looking at.
What about multi-GPU? What kind of expansion cards are you going to be using? Firewire? eSATA? If you're not looking for bleeding edge, things like motherboard slot configurations and onboard chips become more important. I haven't looked in-depth, but from some reviews, I've noticed that Asus often seems to be show better USB and Firewire performance.
I'm pretty sure even the worst of the X58 boards will push an i7 920 to 3.2GHz, although I'd still recommend an aftermarket heatsink.