I have a Dell Dimension 8400 case, what PSU will fit in the box and work with the stock mobo?

IF Dell is stil using proprietary connections for the PSU's then your only option is with PC Power&Cooling. they make Dell compaitable PSU's. I havnt checked Dell's llately but I had heard that they were no longer using Proprietary connections, in which case you can use any retail PSU.

Dell should be able to tell you about the PSU and whether or not you can use an off the shelf one from any retailer.