I have the dell XPS8500 as well, and its power supply although is rated at 460w, I do not think can handle more than 350w continuously at the 12v rail. The motherboard only has a single PCIE x16 slot. The chassis have a HDD frame that will block power connectors to graphic card of more than 11inch.
My humble opinion is to find a card that is less than 10.5 inch long and upgrade your PSU to something better.
I did mine with a gtx 680 and seasonic x660. Fits perfectly with some struggling.
good luck.