I looked at doing this with a Radeon 9200. Basically, besides getting more GFX mem, it was not worth it for me.
http://forums.amigaos.net/viewtopic.p ... d1ae52681d86d9291a66b0856It is doable since I tried it out anyway due to curiosity. From memory you need to change the monitor tool type to the name of the card ie. Radeon 9250. There is also a setting in UBoot you can change to use PCI GFX instead of onboard.
Sorry, I am at work and it was some time ago when I did this. I do remember it started having problems with the later AmigaOS4.1 updates which is another reason I stopped bothering with it.