No to be nit-picky but... Saying "quad 4870 X2's" is incorrect.... you can't put 4 (four) 4870X2's into one computer. You can only put 2, due to the GPU restrictions in CrossfireX. You can do TWO 4870 X2's for a grand total of 4 gpus. According to the reviews and the updated architecture, this might be a better option than 4 4870's.
However, at that point, it's really a question of how much money you want to WASTE, considering that 4 4870's would cost about $800-$900 where 2 4870X2's would cost at least $1000.
But hey if you got the money to throw around and a monitor with the resolution to actually see how much power you'd have (say, oh, 1920x1200), more power to ya. But if you're running anything LESS than that, it would say just use 2 4870's or even 2 4850's. More than enough power, and way better bang for the buck.