CrossfireX is what you would be looking for, then you know the board and card match up well. SLI is Nvidia only, crossfire is ATI only. Apart from that, i have no idea what else would be holding it back. I would hate to see you buy another board and have the same problem though. I wish i could tell you 100%. Its possible that you got 2 bad cards in a row but the chances of getting that are pretty slim. I understand that your hdmi quit so i might be considering a different store to shop from. Are the cards your buying branded by someone particular? Asus, foxconn, MSI? Im asking because that sometimes makes a difference. I've had MSI run like crap and die where i've had foxconn run like a top and keep going. I have a 7950gts foxconn in the wife's machine thats 2 years old and my shop machine has a 6800gt foxconn 4 years old and my game rig is my 8800gt over a year old all top notch.
On the other forums your checking, look into what motherboards they are using. I think you'll find their were some chipset changes to get the most out of the newer cards. Also make sure your running a pcie 2.0 slot, if your still on a 1.1 your only getting half the bandwidth your suppose to be.