hi i was wanting to crossfire my his hd7850 g-card with another one the same but my motherboard only has 1 pcie3 slot and 2 pcie2 slots can i cross fire the cards having one in a pcie3 and the other in a pcie2 slot ? if not would i be better off using them both in pcie2 slots ?
Now that is a very good question. Offhand I would think it would work, but if I remember correctly, pcie 3.0 transmits data differently than pcie 2.0 ... so it may not. I can't imagine it would hurt anything to try, but odds are you're just better off using your 2 pcie 2.0 slots.
From the benchmarks I've seen, single card or even crossfired, barely any difference between 3.0 and 2.0 performance. Most current games aren't designed to take full advantage of the bandwidth 3.0 offers.