Hrmm depending on the power supply. Most dell computers only come with a 250W psu, but they tend to underrate them as in wattage wise.
I had a optiplex 260 with a 250w psu that came with the computer. I have a 9800 pro and two hards and one sound card with a CR-W drive. Well ever since i added the second harddrive and a sound card, i have had difficulties with the psu. Sometimes on boot up, it says that it can not detect my hard drives or my 9800 pro is not connected.
Ever since that i rebuilt my pc with an Antec 550W psu using a different mobo but eveyrthing else was reused. I havnt had a problem ever since. So you might be worried that your psu can not handle your 6800GT since its alot more demanding than my 9800.