Check your current PSU and see if it has a power switch in the back of it. More than likely it doesn't.
PC Power and Cooling designs a line of PSU's specifically for Dells. I've ordered a few of these for Dell PC's and they have all worked wonderfully.
It's 500 watts, has SATA connectors and is SLI certified. $84.99 USD
I know there is something about Dell motherboards that are not compatible with regular ATX PSU's that have switches. It might have something to do with the actual case design. Based on the picture of the service manual it doesn't look like a standard ATX PSU will fit in the case because of it's design.
Trotter might be right about newer ones not needing anything special but I'd contact Dell to verify that.