Here's a great nVidia tip:
Want to overclock your CPU or GPU on your nForce motherboard but your BIOS doesn't have the option? Well, there is a add-in to your nVidia control panel called nVidia System Tools. When installed, it adds a Performance branch to your nVidia control panel, and after you accept the Terms and Conditions, you have a great bunch of useful tools. You can adjust fan speeds, speed up that GPU (I have a GeForce 7300GT card, and I got it to go from 350 MHz to 450 MHz), speed up the system bus (faster CPU!) and more.