You can do it, I did, but success depends on how fast your memory is. Most 5900XTs come with memory that at best will do 800 to 850 mhz overclocked. To run at 5900 Ultra or 5950 speeds your memory needs to do 900 mhz or faster. Most XTs are not equiped with the 2.2 ns memory required to run this fast.
If I were you I would just use Coolbits or RivaTuner and overclock your card as far as it will go while maintaining stability. With any luck you'll hit 450 to 500 mhz on the core, and 800 to 850 on the memory.
Here is link to read about flashing bios and modifying the 5900XT series:
In reading that thread maybe one of their modded bios would work great. I flashed my bios no problem, but I did my homework. If you do it, talk to others who have a 5900XT and have done the flash. Good luck