Overclocking is not an exact science. When it comes to overclocking headroom, every GPU is different. And just because your friend's HD 5870 hit 1000MHz doesn't mean that yours will, even when using the same settings and other hardware. Many factors can influence what a video card is capable of, starting with the 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 and GPU voltage.
To overclock the Asus Matrix 5870, we used the included iTracker 2 overclocking utility from Asus. We found stability was at 973MHz core clock and 1293MHz memory frequency. That's a 14% overclock on the GPU and an 8% memory speed increase. With the higher frequencies, we saw a 9% performance boost across our gaming tests.