Be caureful about the motherboard if you replace it... Dont know about the new full tower dells.... but I've seen some recent XPS Studios with a funky shaped motherboard that kids brought in to have work done on....
the lone PCI Express shouldnt be an issue, if you ever want to SLI it though, you will need to get an ATX.
About the PSU, if its a newer Dell it should have a standard ATX PSU, but if its any older then a few years youll want to double check that its an ATX and not one of Dells funky long, full case 2.5" thick PSU (if they make them still).
Someone criticize if I'm wrong, as I dont know when Dell switched their PSU's to the ATX standard, as well the boards.