what graphics do you have now? what is your budget? it looks like you should be able to support the modern graphics card on the market right now but to be sure, is this your computer: http://www.dell.com/us/en/dfh/desktops/studio-xps-8000/pd.aspx?refid=studio-xps-8000&cs=22&s=dfh? also, do you know what power supply is included with it? you might need to upgrade that as well depending on what gpu you decide to go with.
EDIT: looks like it is only a 350 watt power supply. to get anything new, you'll probably end up having to get a new power supply.
What the hell 350 watts powering an I7??? Holy crap man get that PSU out of that case!!!
I'd suggest a 5770 which is the max of your budget, but since you also need a PSU we'll knock it back to a 5750, which is almost as strong as a 5770 anyway with a good bit smaller price tag.