based on answers above I thought you might struggle to find a decent mATX power supply @ 400w or above.
seems not, according to these folks a standard ATX psu would be fine. http://en.community.dell.com/support-forums/desktop/f/3514/p/19361125/19806917.aspxhttp://support.dell.com/support/edocs/systems/dim3100/en/sm/parts0.htm#wp1354645
I myself have replaced stock Dell XPS 420 PSU (400/500w ish) and that was mATX system. I replaced with a nice new ATX 750w or 850w PSU - went fine even in the limited space I had to work with in the mATX case. be warned that your cable tidying after the PSU swap might not be as good as Dells and if you struggle with temperatures you will want as cable tidy box as possible - just take your time figuring the best method to stop the cables blocking the airflows - youll be fine, check the space requirements for the PSU vs the actual dimensions of a new psu.
something like should be ok for you, please do some more checking yourself...http://www.ebuyer.com/268696-coolermaster-450w-gx-psu-rs450-acaad3-uk
- if I still have the stock Dell XPS 420 psu you are welcome to it - let me know, it worked fine but was not enough for my system with ATI HD 4870 - I live in the UK btw