I'm not sure a lot of people would have released them for the 7800's yet, but you could ask a friend to copy the BIOS from their 7800 GTX so you can use it in your card.
that's one thing I should add, before you flash it, copy your original BIOS and put it on another floppy with a command in autoexec.bat to flash it back, that way you can restore it if something goes wrong.