Yeah, I think Spike is correct.  Even if you could get a bus that could handle so many cards, power is your big issue here.  You're going to need a bunch of computers with their own power supplies to run something like this.  Think of racks of servers.  There's no feasible way to build a power distribution capability (at least not in a reasonable amount of time, since I assume you'd want to start mining quickly before the reward drops by 50% in 2012 sometime).

And of course even if you can get free electricity (just run a jumper from your neighbor's house! :-)  you're next issue is going to be cooling.  You will need a lot of AC to keep the place from melting down, and that will require not just the AC units, but adequate venting and ducting to ensure that the hot air can escape.  Or maybe a bit water cooling rig....   Big project.