Want to upgrade my 305w psu to 450w on my dimension e520

Check the pinout of your mobo's power connector and see if it is the same as the current standard. If it is, you can get most any [quality] PSU that will fit your case. If not, you may be stuck with whatever Dell offers.

Also, if you have the standard desktop case (as opposed to the slim case) you can get any ATX compatable PSU; if it's slim, you're stuck with the factory unit. I'd suggest the Antec Earthwatts 380 or 430, so long as the pin-out is the same.

standard desktop case will check the pinots on mobo

You don't need a psu upgrade for an 8600 GT, if you still want to you could use almost any standard ATX unit, just don't try and stuff a 1000 watt monster in the box.

Correct. As delluser said, you don't need the upgrade. Thats 305W sustained, not peak. It also has 22A on its 12V rails, more then enough for the 8600GT. I wouldn't bother with such a weak card, I'd look into getting the 9600GT, or 8800GS/GT. All three of those cards should work just fine in there, all without a PSU upgrade.

8800 GTS in a Dell Precision 390 with 375w PSU.

I don't remember the exact watts drawn by the 8800 GTS, but it was about 5w more than the biggest power drawing Quattro that Dell offered as a standard upgrade for the Precision 390 and about 8w less than the dual Quattros offered in the Precision 390. I figured Dell would not risk $1000 worth of Quattros for 3-5 years (business class warranties) if the PSU was undersized. There are guys all over the Dell forums that had done the same, so the safety in numbers reinforced my thought process.

That was 1 year ago. Still going strong with Q6600 plus 4 HDs.

Bottom line, Dell seriously underrates their PSU's using sustained power rather than peak power like the PSU manufacturers. BTW, HP is just like Dell. The HP workstations have the same "crappy" 375w PSU with identical Quattro card selection.

The 375W is slightly different. The 305W dell PSU has 22A on the 12v rails, the 375W has 30. It might be a bit more then that, I don't rightly remember at the moment. They do "underrate" their PSUs. The 305W is more like a 400-430W, while the 375 is more like a 450-500W. The 305W is enough to run the 8800GT.

Is the Dell 305W power supply enough for the 9800GT card in a Dell E510 with an intel Pentium D 940 and two hard drives?

The 9800 GT uses about the same amount of power as an 8800 GT, your psu has a few years on it already, it may work but I wouldn't run it for long without upgrading.