Quad crossfire is yielding almost zero performance increase over a single x2. Crossfire 4870 or 4870x2 is going to be the strongest setup you can get. The only time the x2 pulls ahead of crossfire 4870s is in games that actually use more than the 512mb of memory on the crossfire setup. This only happens on 2-3 games on a 30" monitor. That being said, an x2 is probably going to be a bit cheaper than going crossfire 4870 and good board. Also, the x2 seems to draw a smidge less power than the crossfire setup.
<3 my 4870 crossfire setup.