So I stumbled upon this little gem. A Polish Web page called pclab.pl did an extreme overclocing on a Core i7 platform with 4 HD5870 cards. Everything was heavily overclocked and cooled with liquid nitrogen.
They used an ASUS P6T7 WS Super Computer motherboard, a Core i7 975 at 4.75GHz and the Radeons were OCed to 1.315GHz on the core and 1.3GHz on the memory. They also moded the cards physically to give them better power conditions.
Result? A single OCed HD5870 at 1920x1200 AAx8 in Crysis got 39 average (27 min). For HD5870x4 the score jumped to 69FPS average and 31 min... Crazy
Stock number was 31FPS avg.
Link here, all in Polish though