It's not your only option.
You can buy pretty much any standard ATX power supply and cutout a little bit of the case's backplate to allow for the switch.
Why do you want to upgrade to a 400 watt unit ?
I don't think the case is my only issue. I have heard that dells are wired a little differently than normal computers, so you have to becarful of your psu choice.
I would like to upgrade my video card and of course they draw more power nowadays if you want something decent