I did replace my Dell PSU with an Antec 500W and ran into a couple problems, however i did get it in:
1. Dell DOES use ATX PSU, but it doesnt have an on/off switch, so you have to get screw spacers to push the new power supply back a little so that the pressure against the case wont turn off the switch.
2. Dell's ATX PSU's are a little shorter in length than my replacement. There is also a piece of bent metal than kind of "springs" the psu into place when you put it in. That was an issue for the longer PSU, so i had to kinda re-bend it to where it stayed down and didnt push on the new psu.
3. The case with the new PSU can get pretty darn tight. My case right now has 3 HDDs, 2 gb memory, x800xl, and 2 pci cards and it's a freaking oven lol...better to get a new mobo/case if youre in it for the long haul or have alot of stuff making your case hot (i'm going to get a pci fan or a third party GPU fan to see if i can cool things down
So overall, YES it can be done, NO it's not a pop-in pop-out operation
PS: My Dell 8400 mobo used a standard 24-pin connection with the 12v connection for the P4 processor