The BIOS chips used to be bigger, depending on the graphics card. I believe Nvidia cards used 64k code for regular PC compatible cards, and 128k for Macs, while ATi used mostly 128k for both, while sometimes, you would be unlucky to find a card with only 64k, but there are plenty of reduced size ROMs to reflect that, and people would sometimes also just simply solder on a bigger ROM to the card anyway.
EFI certainly does affect it, yes, and part of the increased cost comes from the need to develop a new compatible ROM for the graphics card. Hence the higher cost for the 8800GT as well, and the longer wait until we finally had, and now have, new graphics cards.
EFI will be the norm for our PCs soon, so here's hoping that ATi and Nvidia are prepared with their graphics cards too, but something tells me we'll still be using our current cards with EFI motherboards.
The Mac forums must be going nuts with this, as the 3870 has been wanted for so long for Mac Pros, especially since people have been getting them to work with custom kexts under OS X.
Well, live and learn, didn't realise this:
BIOS chip? video-BIOS is stored on an EEPROM that's part of the GPU die itself. Unless they modified the RV670 itself....
This would make it even easier for a flash, since there's almost no way that ATi will bother changing the EEPROM just to accomodate a different type of BIOS.