But there are Macs with Nvidia cards and you definitely don't need PCI Express to mount GeForce FX card, nor you need PCI Express for GeForce 6 and 7 carda, since there are (have been) AGP models available.
But it's quite easy to find supported models with Radeon cards. And tower models can use AGP/PCI*Radeons so exchanging the gfx card is not such a big problem.
I can bet this will change the day MorphOS will be able to support Radeon HD and GeForce cards too.
If by "this" you mean "this hobby" I'm sure it wouldn't change much even if we had support for latest cards. No Amiga solution will ever be a "serious" alternative to Windows or even Linux.
2D support for RadeonHD is not such a big deal as 2D part seems to be more or less similar to previous versions. Initializing it through ATOM-Bios is relatively easy (although I don't know what is used on Mac). 3D part on R400 and R500 models is AFAIK an evolution of R300 part so if very good R300 support is developed R400 and R500 drivers will have a good basis.
Many MorphOS users have a Mac Mini G4 with Radeon9200 and many more will have Powerbooks with R9700. These are the chipsets that matter right now because these can't be exchanged. Next MorphOS release 3D drivers for Mac Mini are so optimized that will probably match ATI developed ones in performance. I'm typing right now from a Mac Mini so R9200 is the chipset that I care for. R9700 is inside the powerbook next to me waiting for MorphOS and that's the 2nd chipset I care for. Powermacs? I don't think I get one but If I did I could survive with a Radeon9800.
Are you KIDDING or what?
I'm not, as I have explained above high quality R200 and R300 support is more important than supporting R400 or R500 because most of MorphOS users will need R200&R300 and not later models.
I'm not concerned about Warp3D performance at all because Warp3D games used to fly even on old A4000 with CSPPC. Most are designed for Permedia2/Voodoo3. These games fly on my humble Efika so I don't know what's the fuss about that. Quake3 and Wolfenstein already run reasonably fast on my Mac Mini (and don't use Warp3D at all).