Actually, Rivatuner will not avail you much, because the Geforce 8400 GS does not have the connections which allow for fan speed control. This is a problem, because the card DOES in fact have overheating problems on account of its poor fan system. The fan won't spin, or spin fast enough, unless the card is excessively hot. In other words, the cooling systems for this card were just designed poorly.
Rivatuner isn't the only program that won't help you. There is no way to control your fan speed on this card short of somehow installing the hardware. I speculate that the best thing you can do is install a new fan and/or look into fan lubrication.