I really don't think you should bother.
I have dozens and dozens of spare CPU cores available in server farms that I manage (and could utilize no questions asked) and doing the math just didn't add up for me. Even if I throw 64 cores at BTC in their spare time, you're still only really getting 2000khash per core for a total of 128,000khash across the entire 64 core cluster. That's about as much as a single RADEON 5770 worth of computation and you're now dealing with the constant dimeing of your server resources (potentially lowering their lifespan as the fans labour to keep up) plus the energy and cooling (and noise) overhead.
You'd see more return from just buying a cheap-o rig at Best Buy and sticking a couple of 5770 cards in it in SLI.
By all means if a spare server farm is your only cluster go nuts but there isn't much, tuning wise, you can do to make a Xeon keep up with even a trivial onboard GPU.