Two of them will definitely fit in but powering them is the problem since there are only two six pin PCIe power cable sockets. You could use SATA to six pin PCIe power cable adapters going from unused drive sleds, optical bay etc but I know some people using this method had stability issues. Others used a second power supply - you can get special ones that go into 5.25" bays and I know some people on here used them with dual 4870s. The risky option is to get two 6 pin PCIe power cable y-splitters. You'd be taking more power from the board's 6 pin connectors than they're rated for but it could be ok. FWIW I used this method with my dual 4870s that I used to have in my Mac Pro and I ran them like that for about 9 months before upgrading without any problems and 5870s are meant to use a little less power than 4870s. Still, you run the risk of blowing the tracers on the board.
