Doom3 runs worse with better video card

Hi this is my first time in the gaming software area of VDR.
I'm running Win XP Pro SP2,
Gigabyte K8 Triton, Nforce chip set,
AMD 3000+ 64bit CPU Socket 939
512 MB DDR RAM
At the time I purchased Doom3 I had a GeForce MX400 64MB Video Card.
The game ran OK in the lowest settings.
One day I decided to purchase a GeForce6200 128MB DDR Video memory, AGP 8x. I thought it would help the game run smoother. I uninstalled the MX400 drivers and shut the computer down. I removed the old card and installed the new one. I booted up and it said found VGA Controller.
I ran the NVidia disc and installed the drivers.
I restarted and everything seemed ok until I tried to play Doom3.
The game either goes to a black screen during the opening video or freezes in the game usually not too far in. Also now everybody has a line down the middle of them. The game looked and ran better with the older card.
I've tried uninstalling and reinstalling the video drivers. I've installed the newest driver. All with the same results. I took out the card and put my old one back in and the game runs but everybody has a line down the middle of them that wasn't there before.
Any ideas what went wrong? I also returned the card and got a new one off the shelf.
It also freezes Call of Duty, and Castle Wofenstein, where the 64MB card didn't.
Thanks in advance, sorry for being so long winded but I've been at this for a few days now.
Dave.

What's the power supply like in that machine? Do you know what sort of temperatures you're getting on the CPU and video card?

Did you try reinstalling the game and applying any patches available for it?
BTW you could try Omega drivers which have been optimized for gaming.