I've had the same problem OP. I'm using a 5670, and my GPU got to 90c with the fan on automatic settings, and 80c with it set to 100%. I bought a PCI cooler to put right over my GPU to suck away the hot air hovering in the top of my case (my case only has one exhaust fan on the bottom, under the GPU) and I mounted an intake fan on the front of my case. That, and doing some cable management keeps my GPU at 75c max with the automatic fan setting. Is your case cooled properly?