NVmax and Coolbits
The easiest and simplest way to overclock your video card is through an application in Windows. These will also give you the best speed gains compared to the BIOS and other methods by a long, long way. The two main utilities used to overclock nVidia graphics cards are the program 'NVmax', and the registry hack 'Coolbits'. While these two offer the same basic tweaking, that of the core and memory, NVmax also offers a myriad of options from filtering to general refresh rates. Thus NVmax is far more powerful, but Coolbits still gets the job done. Obviously the best place to start is to download one of these programs. The choice is yours, but I'll explain what to do for both versions, so get both if you feel like it and experiment to find what's best for you.
To download Coolbits, go to this URL
To download NVmax, go to this URL