Notice that we haven't verified this information however you may still want to give this a read and see if this sort of modding can be applied to your GeForceFX board (spotted at W2S):
The guys over at Madshrimps have dug up some info for flashing your nVidia FX 5900 non-ultra to a fully fledged FX 5950 Ultra graphics card!
nVidia used to hardwire all graphic cards with the correct BIOS so flashing cards like ATi wasn't possible... However, newly released information suggests that the NV38 (5950u) is simply a software rebadged NV35 (5900).