Actually, I'm kind of bummed out now. I may just return Doom 3 (never opened it). I checked some reviews at anandtech.com which cover newer video cards and how well they perform with games like Doom 3 and Half Life 2. While Half Life 2 is very well scaled to work on both state-of-the-art systems (high end CPU and video cards) and mid to lower end systems, Doom 3 is not. Doom 3 is heavily geared towards the very high end systems, and apparently doesn't run well on mid-level stuff. Remember, I only have a 1.8GHz CPU, which is barely above Doom 3's minimum of a 1.5.
According to the review I read at Anandtech, a 9600XT will only give about 25FPS on this game, and that is with a much higher end CPU (I think an Athlon running at around 2.5GHz). Which means that it will run like crap on my system, even with the 9600XT installed.
BoriquaSNK suggested (wisely) that I pick up a 9800 Pro card instead of the 9600XT, and in fact the reviews at Anandtech do show the 9800 Pro as being well above the 9600XT in terms of performance (probably due mainly to the 8 pixel pipelines on the 9800 vs. 4 on the 9600). But the 9800 Pro is $250, and the 9600XT was $100 after mail-in rebate. Bit of a difference in price there, and I'm trying to minimize the $$ I put into my old system since I'll probably eventually build a new one.
I'm wondering whether it is even worth upgrading my current system much at all. The Gigabyte motherboard maxes out at 2.0GHz, so no room for growth in the CPU. I may just drop the 9600XT into the system (haven't installed it yet, still running my old GeForce 3) to see how my older games run with it like Serious Sam 1 and 2, UT2K4, and (MAINLY) Painkiller, which chugs a bit on the GeForce 3.
As far as Doom 3, since I haven't even opened it yet, I think I'll just return it. Its ridiculous that they geared the game so high, too bad it wasn't designed more like Half Life 2 (which I WILL get soon). Personally I think FPS games can be tons of fun without having every bell and whistle enabled, look at the Serious Sam games; fantastic graphics (but not state of the art) and great fun to play. The Serious Sam port for the XBox was even fantastic, I got tons of hours of play out of it and the control and graphics were spot on.
Oh well, lesson learned . . . If I don't build a new rig every 2 years or so, my existing one gets stale for new games.