It is usually true in saying that NVIDIA cards cost more, check out these prices. I have picked the cheapest 6800 Ultra and cheapest X800 XT from newegg.com
PowerColor ATI RADEON X800 XT Video Card, 256MB GDDR3, 256-Bit, DVI/VIVO, 8X AGP W/ Limited Assassin Edition Backpack, Model "R42-TVD3" -RETAIL
CHAINTECH nVIDIA GeForce 6800 Ultra Video Card, 256MB GDDR3, 256-bit, Dual DVI/TV-Out, 8X AGP, Model "AA6800UA1" -RETAIL
I didn't want to cheat so I just went straight onto newegg.com. These were the only two available though.
256MB AOpen Aeolus GeForce 6800 Ultra £358.96
256Mb Connect3D Radeon X800 Pro
They had an XT version for "Call for price". I would probably say they are the same price.
NVIDIA are getting smarter