Dell NEVER used ATX power supplies, at least not until recently. And the recent systems use proprietary board form factors.
The earliest ATX style Dell power supplies were found in their Pentium 1 systems. They had the ATX power plug, but wired differently.
They later changed to a two plug power supply, with a 6-pin AT style power plug and an ATX style power plug. It looks like the same power supply used in industry standard NLX form factor cases, which had an ATX connector for the board and an AT style connector for the riser card. But AHA, it isn't, the ATX part is still wired wrong. That is to say, an industry standard NLX power supply will work on an ATX system, but the Dell power supply which looks identicle...wont. Dell introduced this style power supply around the same time as their original clamshell style Optiplex systems came out, around the time of the Pentium Pro intro.
I've heard from 3 sources now that Dells latest 8800 systems use standard ATX power supplies, but I know as a fact the board is a proprietary form factor, the "ATX backplate" is longer than standard.
<font color=blue>Watts mean squat if you don't have quality!</font color=blue>