Overclocking is not an exact science. When it comes to overclocking headroom, every GPU is different. And just because your friend's GTX 480 hit 900MHz doesn't mean that yours will, even using the same settings and other hardware. Many factors can influence what a video card is capable of, starting with unique yield characteristics of each GPU. Other factors may also include complementary components within the system, but we find the primary influence depends on the type of cooling used on the graphics card.
To overclock the GTX 480 Amp Edition, we used Afterburner overclocking utility from MSI. Stability was achieved at an astounding 845MHz core clock and 1025MHz memory frequency. That's a 12% GPU overclock over the rated speed of the Amp Edition video card, and a 20% increase over the reference design.
Magnifying the overclock found on the Amp Edition might seem excessive for some, but taking the time to get every ounce of performance from it can be rewarding. We were able to produce a 14% performance hike with our new settings.