Hey Hulkster,
I went with the "system recommended settings", which set me at medium detail and 800X600. I haven't really played around with the settings much to see how high I can take it before I notice any slow down (will try that tonight, at least bumping up to 1024x768), I also have almost everything turned on (In the advanced setting tab there are check boxes, I believe everything is checked with the exception of anti-aliasing). Again, I'll play around with the settings and see what kind of drop in performance I notice. It does look FANTASTIC though. One problem I seem to have (don't know if anyone else has seen this or not), is that occasionally my sound will go away. I'll be playing and it will get silent (like it is being turned off)...if I go to the main menu and disable and re-enable the sound it is good again...it only happened twice last night (once in the beginning cut scene and then once during game play), just can't figure out why...I'll have to check their website and see if anyone else encounters it. I'm using a Sound Blaster Audigy card (the first one), with the latest drivers..so who knows.
Oh one other thing I should mention, I read in the latest version of PC Gamer, that Doom 3 is more dependent on your graphics card then your processor, so I would imagine with a 9800pro you should be able to run things very well (although for the ultra settings I would imagine that would require the latest ATI or NVIDIA cards (x800 ATI?? or the 6800 Ultra cards).