Power supplies are pretty reasonable right now. For a few years I was making the mistake of getting cheap PSU's for like $14.00 for 400w. I've since learned after replacing them every year, it's better to spend a bit more and get a decent one.
 Here is one that should do the job if you find yours is not sufficient  Corsair PSU

 For the most part, Dell uses some pretty good quality PSU's and I think it's likely you will be OK with what you have.