Knowing Dell, installing a new PSU will probably be hard work, since there cases are mega crap. Last year I was fitting a new PSU in my Dell case and wotcha know, it didnt fit. There was no hole in the back of the case to allow the PSU switch to fit properly, and the manual fan adjusment was in the way.
Dell is dumb, just becasue there PSUs are switchless doesnt man others are.