2X ATI Radeon HD 6450 1GB DDR3 or 1X NVIDIA GeForce GTS 450 1GB GDDR5?

Hello,
I am getting a custom designed gaming PC (http://www.vibox.co.uk/vibox-destroyer) for all the specific details.
and I would like to know if it would be better for me to get 2x the Lowest ATI graphics cards or the midrange NVIDIA card stated in the title, I will be looking to play games with graphics such as Guild wars 2 or WoW as I have my Xbox for the higher range games.
I am very uneducated when it comes to this sort of thing so any advice would be appreciated.
Thank you.