I have a Geforce 260 and a 560ti. If I remember correctly lowering the memory clock on the 260 lowered the hash rate. Lowering it on the 560 (from 2004 to 1000) doesn't affect the hash rate (I'm not sure about the affect on temp yet) but if I stop and restart my miner it will fail to reallocate CUDA memory and put set the card to idle speeds until the computer is restarted. I'm not sure yet if this is a driver problem or if the card really doesn't like such a low memory rate.

Regarding the temperature of the 260 - mine runs very cool - 54-60 degrees C uner 98% load compared to ~70-80 Degrees for an overclocked 5830 in the next slot. Perhaps the fan needs cleaning? Also, are you setting the fan speed manually? Try ~50-60% (or more if your ambient temp is high), this should lower the temp significantly compared to auto.