68C is perfectly fine for an 8800 GTS. Some people run them up to 90C without having an issue and the radeon 4000 series can run hotter yet. Your fx 5500 is a completely different graphics card designed to run differently, its temperature threshold cannot be compared to a more recent card.
OP, If it keeps happening, try checking your video ram temperatures instead of the core temps. GPU-Z can tell you that information.