Chances are, from the cards available CURRENTLY, I'm pretty sure the game will look best on the ATI 8500...and besides, didn't Carmack's last .plan said he's done the work for the 8500 code path?
Well, in Carmack's latest relvant .plan, he basically said the following:
1) Radeon 8500 is a "fine card" for Doom 3
2) There was some technical issue that he felt was holding back Radeon 8500 performance from realizing it's potential. He's not sure if it can be worked around or not.
3) He was "guessing" that the GeForce3 would be a "safer buy" for Doom3.
Now, that was quite a while ago before several driver releases so who knows if things have changed. However, based on what he said back then, I would conclude that the Geforce4 would be the best of the "current cards" to display Doom3 on.
Though for all we know, ATI will launch the Radeon 8500 successor (R250?) at e3, and maybe it will be displayed on that.
I am hoping that Doom3 is displayed on either the Matrox or 3D Labs part though...because that will give some instant credibility to the GL drivers (as far as gaming goes.) The current rumor surrounding Doom3 is that it will be shown on a "multimonitor" rig...add that to the Parhelia triple-head rumors, and the "Doom3 will be shown on Parhelia" rumor is born.