Depends which model. If you have a BTX-styled Dell, then just get an ATX/BTX PSU and that will work fine. I believe those systems have open backs so you don't have to worry about the switch.
If you have a Dimension 4600 or certain other models, an ATX PSU will do fine (since there is an open back for the switch). Certain older models used proprietary PSUs, so you'd have to look at going with PCPower and Cooling for a compatible PSU. Also, some older models can use a standard PSU, but you'd have to cut open the back a little for the PSU switch.