Yes, there are now two revisions of the 7900GTX - and the BIOS's are quite a bit different. I had modded another user's revision 1 GTX BIOS sometime ago, but I can't remember where it was on the forums and I don't have the file available anymore. You definitely do not want to flash a revision 0 GTX BIOS to a revision 1 card - namely because the file sizes are different.
Noting that, I am not 100% sure, cancel an even drive either of the 7900 cards to 1.32 volts. If I recall from the days of overclocking with the GTX BIOS - I do not believe that the voltage regulator supports anything higher than 1.24 volts on these cards. I have not looked at the PCB layout on a 7950 GTX card, but I have a feeling the voltage regulator would have a different part number.
Theoretically, if you could drive the 7900 series cards at 1.32 volts, you wouldn't have any trouble at all overclocking to 7950 GTX speeds. All three cards (7900GS, GTX, or 7950GTX) have the exact same core chip. None of them are cherry picked, they are all the same G71 core, but ran at different voltages. As a general rule, if any revision is made to a GPU chip that revision would be reflected with a new numbering convention (a.k.a. G70 perhaps). In this case even if they had revised the G71 chip and kept the G71 name, then that G71 chip would be used in all 7900 series cards. Same core, different voltage.