Unfortunately ATi never post BIOS updates on their sites. You'll have to download OEM versions of the firmware from their partner sites. There are quite a few (5 or 6) that do have firmware updates.
I performed the exact same research for the exact same card (more or less: ATi Rage Fury) a few months ago.
Strangely enough, ATi uses flash EEPROM chips. I managed to find a copy of the flasher at a partner site.
The partner sites are listed at ATi's website.