a. is it AGP or PCIe card?
b. are you planning to use it in PowerMac?

If it's AGP and you're going to put it in a G5 PowerMac then theoretically it should work with MorphOS, even if it's got no Apple ROM - and I can assure you it's got PC ROM, because there simply weren't any Apple AGP OEM Radeon HD 3850's out there (no Open Firmware, no OSX and no Apple boot menu though). But if it will work - that's another question (depends on your luck, really). I'd give it a shot nevertheless.