I'm not positive, but I believe most Dell cases use a non-standard standoff pattern so you can't fit a normal ATX motherboard. They even do silly stuff like make the power connection cables with different ends so they don't work with a non-Dell motherboard.
The stock PSU is most likely crap also. Upgrading an OEM computer is usually not as easy as you might think.
Someone correct me if I'm wrong. (I haven't had to deal with an OEM computer for quite a while