I was running 2 - 5850 cards, played almost all games at decent FPS. average of about 40 FPS on most games, But then I started playing Far cry 3 and no mater what I did could not get better then about 15-20 fps even droping Rez down to 4800x900, game constantly crashing in DX11, 50% the time in DX9. The cards were starting to get to hot alot of the time in that game as well.
I just got my self a hd 7970 last week, I can say was a great upgrade. I now get in FC3 35-40 FPS in eyefinity 5760x1080 every thing on full, sweetfx on and and running in DX11, game has not crashed since. in Planetside 2, was getting 20-30 FPS in heavy battles, now getting 50 FPS. AMD though has an Optimize out for Crossfire that on my 5850's boosted FPS by 40% so I may get more if I was still running crossfire. A friend of mine over holidays picked up 2 - 7870's and is now getting 80-90 FPS in PS2 in heavy battles.
So it would be worth it, in my mind now. I was leary at first as it was ok on most new games, bearable FPS. But now I feel it was a good move, as I can do alot more with my card and set for some time again