There’s no info yet about overclocking experiments on the 5900XT with extreme cooling, which would show “the limit of the card’s abilities”. By my personal estimates, such frequencies would be somewhere near 700/1050MHz.
The record for now is 650MHz GPU (Vgpu=2.0V) and about 1050MHz memory (Vmem=3.43V) without BIOS re-flashing. Actually this is my own result, as I held a world record for 5900XT performance in 3DMark 2001 SE (27229 points).
(source: Xbit labs)