Get an ATI card. The 5870 is pretty much a GTX 480 or an ATI 5970 which is a lot better.
ATI cards have mini-displayport which can drive an apple monitor. The alternative would be buying an active min-displayport to DVI adapter for $40. Quite expensive for just an adapter so ATI cards are probably a better bet for your money.
Im also surprised you didn't get a 5970 since you are getting such an awesome computer.
The Mac displays are very nice IPS panels.