The naked card (Asus) I bought back in 2007 new from Newegg. It has been running largely at stock since purchased except for a few times I overclocked it with CCC just for curiosity's sake. Before I even put it in my computer, I put a Swiftech waterblock on it. It has never seen temperatures above 45c. The second card I got from a forum member relatively recently. Since then they have both been running in crossfire without problems. I guarantee them both to be stable at stock clocks.
The HD4870 is somewhere between an HD7750 and an HD7770 in terms of performance, though these cards only support up to DX10.1. Crossfire scaling is excellent in Catalyst 13.1, greater than 100% in some games. I'm selling these cards so I can afford a 78xx or 79xx, not because I need more performance but because Crossfire doesn't work unless games are run full-screen.