Dell XPS 8500 work with NVIDIA GTX 660?

Hello,

I'm just wondering if the Dell XPS 8500 model with an NVIDIA GT 640 would support the NVIDIA GTX 660 later on if I wished to upgrade it, and how easy is it to install.

also would I need to install new PSU because the GTX 660 does come as an option when buying so Im guessing it will work.

Any help on this would be appreciated as I'm new to all this Tech computer talk :)

Thank you