socket 775...
Lots of boards available. Some $50 for one like this:
http://www.asrock.com/mb/overview.asp?model=g31m-vs2
But from the looks in the manual it seems Dell's using a special made formfactor.
You may have to rearrange the standoffs... and maybe the openings on the back...
Pics please as you go along...
Personally though, I wouldn't waste neither money nor time on a processor and/or equipment from 2005.