You can only run 2 different cards if their in the same series, IE 5xxx or 6xxx
The problem is that every different series of card uses a different architecture, 40nm,50nm, ect and the drivers simply don't support 2 of those cards working together
Not true. My Windows 7 box is happily mining with a 4850 and a 5770.
What miner are you using?
I just started with two 5850's (non crossfire) and can only seem to get one recognized. The other sits idle. The motherboard I'm using does not support full crossfire and causes stuttering in gaming with xfire enabled...so I just disabled it thinking mining would work with both and I could still game on one. Using Phoenix on win7 x64.
Thanks!
You can try using dummy plugs.