I'm building myself a new machine and am not sure what gfx card to go with. My current one is a 6800GT and the two im looking at are the BFG GeForce 7900 GT OC 256MB and the HIS Excalibur ATI Radeon X1800 GTO TURBO. There is £40 difference between the ATI and Nvidia card but i don't know how they compare performance wise, which would you choose?