You can either use the Coolbits reg hack which puts an overclocking menu neatly into the Graphics Driver propeties. I like this one personally as it functions well and is easy to use and is embedded into the driver.
Another option is Rivatuner which is a standalone program that has a lot more features but doesnt integrate so neatly.
In my opinion start out using the Coolbits Hack and if you need more options move onto Rivatuner.
Core 2 Duo E6400, DFI Infinity 975X/G, 2x 512mb DDR2 667mhz, Albatron 7900gt, WD 200gb SATA, Samsung DVD-RW, Silverstone ST-50EF 500w PSU.