Thats true, they do use different sized ROMs, and then lock them out to that size. They CAN be cracked though by manipulating some resistors. That will unlock a PC card so that the larger mac ROM can be used. Really though, is it worth it to only move up to a 7000, move up to the 9200! You could also try to find an original Mac ATI Radeon PCI. That card kicks the 7000's ass. Check out Bare Feats video archives. By the way RAM size does not speed up the card, it only enhances the visuals. Good luck finding your solution.
-Maestro