The best Gaming system that Apple offers, as far as I know, is the Mac Pro with 2 Radeon HD 5770. Roughly equaling a single HD 5870 in specs (maybe slightly better depending on the games). But they do offer the Mac Pro with a single 5870 too. So the choice they offer to take the 2 cards road is kinda useless. ¬_¬
Going the single card route is also more energy efficient.