http://www.extremeoverclocking.com
http://www.guru3d.com
The forums on those sites are helpful for what you want. Rivatuner is a good overclocking program for NVidia cards. Faster memory that is 1gb or larger would do more for you than overclocking your VC, you might not even get a gain if your memory is the bottleneck. Just download Rivatuner from Guru3D and read some tutorials.