Both ATI nd Nvidia overstate the wattage needed for their cards because of the fairly large number of cheap or poor quality powrsupplies out there, these units do not deliver enough power (amps) on the +12v line or lines.
Dell units deliver a very high proportion of their output on the +12v line or lines, so although the Watts may seem low the all importaint +12v amps is high. The Dell 305W unit has as much if not more +12v output than many cheap 450W units!
That said, I would not advise running a HD4770 on it. Stick to the 4670.