I have issue with 5xxx and 6xxx cards paired with 7870 in my setup, the problem is guiminer would reject the shares from 5870 cards saying it's hardware failure, did a search on goggle and found that lots of people are having same problem, I ended up with building another rig and throw the 5xxx and 6xxx cards in, and use the first rig for 7870 and 7950s, I think it's the latest drivers for 7xxx cards are causing the 5xxx and 6xxx cards to get wrong results, this is definitely a driver problem. I'm currently using 11.11 for 5xxx and 6xxx, 13.5 beta for 7xxx cards, both combinations have been working with no problems for a few weeks.