Well, my old 4870 had a massive heatsink on a set of custom voltage regulators from Sapphire, but even at stock, at load (not full load), they would sit around 90C, and at load, jump to 110C.
GPUs and their components are generally meant to withstand high temps for prolonged periods, just the way they are made. In all reality, keeping the temps less than 100C is all you need to worry about, but I wouldn't suggest it. Just because they can handle that temp doesn't mean you should keep it there.
If you're fine with your current performance, then I wouldn't push it any farther. 74C and 81C are great temps for an overclocked 4870.