I feel your pain, I have an nvidia as well. I found that GUIMiner especially hates NVIDIA. I've tried diablo, bfg,rpc miner and guiminer. I find that for Nvidia BFGMiner and rpc CUDA Miner work almost the same, though bfg just feels more stable. I am running an nvidia 570 oc to 900 stock voltage runs about 150Mh/s to 175Mh/s.
BFG:
https://bitcointalk.org/index.php?topic=78192.0RPC:
https://bitcointalk.org/?topic=2444.0