Looks like it's a mid-tower, so it can handle a full height graphics card.
Looks like it only has a 350 watt power supply.
I cannot find a definitive answer as to whether or not you can replace that with a standard ATX power supply.
There is a 450 watt Dell Power Supply for your computer.
Does your computer currently have a discrete graphics card installed in the motherboard expansion slot?
-Wolf sends
Edit:
Dell Studio XPS 8000 - Specs Sheet