Build your own PC. It's not hard or problematic at all, and you end up getting exactly what you want. I don't mean to start a flame war here but Macs are way overpriced and overhyped. They aren't the holy grail of computing by any means, no matter what their advertising / fanboys lead you to believe. You can build an insanely powerful desktop PC for (relatively) cheap these days. Just follow the advice already given here and you'll be golden. newegg.com is where to go.
MPC 1000 w/ JJOS - Korg ESX - Korg EA-1 - Ensoniq EPS 16+ - Waldorf MicroQ - Roland Alpha Juno 2 - Yamaha TX81z - Mackie CR1604 - random rackmount and pedal effects