Had a look around the Strangedogs forum, seems to pretty much confirm what I already knew. There is one ROM, the Apple Computer/ATI Radeon 7500 32mb ROM. However this isn't interchangeable with the 64mb version.

So, looks like I have another useless card. First the Chaintech GeForce FX 5200, then the Gainward GeForce2 MX 64mb and now the ATI Radeon 7500 64mb.

Can I ever get an upgraded card for this?

Probably not.