iMac over heating

Hi, after recently switching to a new iMac (3.1 Intel i5, 8gb ram, running Lion 10.7.4) I’ve noticed that after only a very short time of using cubase with minimal amounts of plugins the back of my iMac gets ridiculously hot, almost too hot to touch. The temp monitor I’ve installed is showing the graphics processor temp diode reaching around 65ºc. Ive installed smc fan control but this doesn’t seem to be very effective, even with all the fans turned on high. When looking at the screen its the rear, upper right corner which seems to get the hottest. Does anyone have any experience of this or any suggestions as to the best way to cool my mac.