Whats the best GPU for a 305watt PSU??

Dell underrates their PSUs. The 305w model can actually handle an HD5750 or HD4770. If you insist on EVGA and therefore Nvidia the best would be one of the newer 9800GTs without a power connector, although it is a weaker card than the other 2 I mentioned.

+ 1. Go ATI.

Really?? I have a 9800gt on another computer and It calls for a 400watt or greater. I dont want to burn up my psu.

Also you say ATI is better? Ive always been an evga fan.

Here you go. See for yourself:
http://www.techpowerup.com/reviews/Powercolor/HD_5750_PCS/30.html

Yeah. The 9800GT without a power connector can even only theoretically use 75w because that's the limit of the PCIe port.
Those cards are under clocked a bit however so they are slightly weaker than what you see in the chart dragon posted above. Another factor to consider is the case, Dell uses some funky ones so make sure how many slots the card can take up and how long it can be before making a purchase.

yea i the case is big but i can only fit a single slot card on the mobo. will an 8800gt work?

what is the fastest single slot card that will run on my 305 watt psu?