Most of the Dell systems have custom mobos - they aren't ATX/Micro-ATX....do add to the headache - the case has custom connectors for power, reset, lighting and any other front panel connectors.
I would suggest buying a new case....and then get a standard ATX or Micro-ATX mobo to go with it. The CPU is probably too old to game with....so you are left with the PSU....which probably won't support the newer tech (unless it is less than 3 years old). If the DVD drive is SATA (not IDE), you may be able to use it....but in 2006 quite a few were IDE.
Good news - the fans may be able to be re-used. If the fans are noisy - you may have to replace them as well.....Keyboard & mouse can be used....