Read this thread at Guru3D.com:
And this thread here at NVNews:
It has lots of info on the flash. Make sure you do much research. With 5900XT, you may be able to get core to 500 mhz core with flash (or faster), but if your memory is slower than 2.2ns (most 5900XT's are) you will not be able to get much more than 800 mhz on it. This is why they made modded bio'es for the 5900XT series, so core would be faster than a 5950, but memory would stay within spec.
Again, do your research. I modded the bios I flashed my 5900NU to for faster memory timings, and its been working great for months. Good luck, post back how things go