You need an overclocking utility. I recommend MSI's Afterburner
The GTX 480 is clocked at 700mhz core and 1848memory with 1038mv standard voltage.
I have a pair of them water cooled and they run at 100% usage all day long at 800 core, 1848 memory, and 1100mv.
A few warnings regarding the GTX 480:
1. It is an IMMENSELY hot and power hungry card. Still powerful as hell, but equally as hungry. Make sure you've got a good power supply to back it up
2. It will get hot. Very hot. It gets hot at stock, so overclocked your fan is going to be much louder. Keep an eye on Afterburner's temperature readings. Anything over 95C is BAD
3. Overclocking can damage your hardware if done improperly. Understand you're jumping into a hobby with a tad bit of risk.
The basic premise is to set the core and memory clock to whatever level of performance you want to get to, then set the voltage as low as possible to get there. Running Unigine Valley is a good way to check for stability.
This has been a very quick-n-dirty guide, but it's a good place to start. Have questions? Post away!