I had a xfx 7990 2 card wide version my problems was keeping it cool. Just keep an eye on your temps because I think I warped mine and I had to RMA it. I was able to undervolt with msi afterburner. Stable I was able to run mine at 1450 mhs @99C. 
I don't try to make it most from it, despite it is running in the cooled server room. It runs with average about 1200 KH/S (scrypt) and temperature around 81-83C. I just checked pool and it shows 1594 in the particular moment. I noticed that sometime load of the card changes and card load shows only 90-91%, of course when its happened hashing speed falls pretty much, temperature also falls up to 69C. I had a feel that it is pool problem. It does not looks like throttling, cause temperature is not high. And sometimes speed is really up (just checked again and it shows 1440). Also I noticed that speed falls when I open TeamViewer, but probably it is coincidence. Memory on board is 4GB and I did not noticed any improvement after 2MB.