Dual Video Cards ... ATI or NVIDIA?

Looking to replace my internal ATI RADEON XPRESS 200 Series 256mb to get better quality as well as speed at ... at high resolutions

The reason I ask ATI or NVIDIA is because I was a big fan of NVIDIA until my computer came with the internal ATI, I've had no problems with either...  is it just  Pepsi/Coke ?

I'll tell you what--I just have one Nvidia card, a 7800 GTX with 256 MB. It kicks ass.
I'm running 2 monitors, and FEAR runs at high settings with EXACTLY the same FPS as it did before one. Adding another card would just make the little girl fucking jump out at me.
I have had NO problems with Nvidia cards, ever. No, I haven't used ATI, but I can't see it getting better than what I have now. I fucking love it.
I'm planning on getting another so I can run a third, hell, maybe a fourth monitor, but I'm not sure if I'd use SLI often--you can only run one monitor under it.