That card you want requires a decent Power Supply.
I can't seem to find any information on the powersupply that is used in that PC.
Though by looking at the GPU already included in the setup i bet its not enough.
It has an 460 watt PSU which is under spec for that card I believe. As well, Dell machines typically come with exactly the cables they need; it is probable that there is no PCIE power cable to plug into a new video card.