If you Google up a review of the actual, official ATI Radeon 7000 Mac Edition you’ll see it has a PCB with an SVideo, DVI, and VGA port filling up the back panel; I remember the PC retail editions looking identical so I guess I’d probably prioritize that one, but alas I have no idea how much the differences matter for flashing. If they worked the low profile Dell OEM cards that used a splitter to break out that weird DVI-ish port you find on those business class machines into two heads might be attractive for a Cube installation? (Compact and it’d make the monitor plugs more accessible.)
I've found a lot of Dell Radeon 7500s but I don't know if they have a ROM large enough to flash Mac firmware onto.