Sadly it's not looking so good. I know this may be heresy on Tom's site but over at AnandTech.com they reviewed Doom3's playability on various cards...
http://www.anandtech.com/video/showdoc.html?i=1821&p=20
Anyways, if you are running at 1024x768 No AA and no AF at medium quality on a P4 3.0GHz with HT
A GeforceFX 5200 Ultra only managed a paltry 37 FPS while the new GFFX5900 spanks the 9800pro.
BUT if you decide to enable antialiasing at medium quality...with either the $500 GFFX or the $350 ATI, you are going to get just about 40-50FPS MAX and forget about any res higher than 1024...(it's interesting that the ATI handles quality settings better than the new 5900...I guess the NVidia drivers are really a problem).
Even then...for the 5200 to be barely playable is a bit disconcerting...
I really don't want to have to buy a graphics card that costs more than my entire system to launch rockets at a cyber-demon...=(
Half-life 2 will probably be better anyway...=)