I have a pair, but I had to disable CrossFireX. The 5970's are a bit quicker, but the 5870 seems more than capable of running just about everything out there.
As long as they run Starcraft 2 and Diablo 3, I'll be happy.
If Blizzard sticks to the way they've done things in the past, those games will run on just about everyone's rigs. They seem to be able to make 'em look nice but not push customers to high end hardware.