Well some programs will be accurate and some won't. Usually the one in the ForceWare Control Panel is pretty accurate, so just go with that one. Even then, 77C is pretty darn hot for idle speeds.
Find some time, take the card out, do a maintenance check on it. That means take the heatsink out, check to see if the fan or the heatsink fins are clogged with dust. After you do that, clean the old thermal paste from the card and the heatsink contact plate (just use tissue and dab it with rubbing alcohol), and when it's all clean, apply thermal paste. Make sure you don't apply too much; just a dot the size of a rice grain is the maximum you should go, or it'll insulate the heat, not conduct it. NOTE: Make sure you use Arctic Silver 5 for the GPU core and Arctic Ceramique for the memory, or just Arctic Ceramique for everything. Arctic Silver 5 is conductive, so it'll short out your RAM.