The Graphics card you selected consumes 116 Watts max power. The Thermaltake Litepower 500w will meet the power requirements. Dell conveniently omits mentioning the PCI slot power supplied by through the motherboard. I know this power is 75 watts on lesser Dell systems like the Optiplex series which I have.
Go ahead, you have a good chance of getting this to work and a call to Dell tech support may confirm it.