> I don't know what AtomBIOS is.

There's a dedicated thread here on MorphZone.

> I've seen some secondhand PCI Radeon 9200 available. Would they work?

No. Radeon 9200 is R2xx GPU generation. While AtomBIOS started with R4xx GPU generation, the AtomBIOS version required for MorphOS is to be found on Radeon cards with (at least) R5xx GPU generation (Radeon X1300 and above).