*Note* I am not talking about sending it into dell to upgrade... I KNOW they do that but it would end up costing $100 + the cost of their cheap/overpriced PSU.
I really... am regretting accpecting this computer as a gift now.
One thing to consider, Dell's case are usually doesn't support 3rd party PSU. Not sure if they change that or not. Make sure your new PSU can be return for refund if it doesn't fit.
It sounds like standard ATX PSUs will fit. A3SEMbler mentions he used a BFG 550w, but needed to bend the mounts a little.
http://en.community.dell.com/support-forums/desktop/f/3514/t/19269320.aspx