I use nVidia 8600GT. Aren't there miners with specific commands that let You limit the usage of CPUs and GPUs
nVidia....
You will need a CUDA miner, not the usual OpenCL ones. For CUDA I recommend puddinpop's
RPC Miner, witch are also included the easy-to-setup
GUI Miner.
To limit the GPU usage you have to set the extra flag:
-f250