You may want to hold off a month or two and see what happens with ASIC's before you spend any money on a rig built specifically to mine with.  There's a chance you'd never make your money back if ASIC's really jack up the difficulty.  I've seen estimates of anywhere from 20-50x, so imagine you're making 20-50x less BTC with the same power/equipment cost and see if that's still worth it.