> It came with a Radeon HD-5770 (it was used in this Mac,
> so I assume it is a Apple version?).

There's no PPC ROM for anything above the Radeon X1xxx series, so neither OpenFirmware nor Mac OS can work with it. Was this PowerMac G5 used with Linux maybe?

> Is this a good card for MorphOS?

No 3D and no overlay for now.