No wrt the 4870, any card you use needs to have a PPC compatible ROM. That basically leaves only things that either Apple or another company sold for use with the G5s.

But a good card IIRC is the GeForce 7800GTX.
I am not sure how it stacks up with the Radeon X1900XTX though.

barefeats might have some benches and you can probably find some used ones for $50-$100 from PC users.


*Edit
Being perfectly honest, what you want to do is a big waste of money when you could just play DOOM 3 on even a semi-modern PC and max it out.

That being said, I know how appealing maxing out an old system is so I just want to make sure you really want this before doing it.