funkb - I cannot help you re the new Dell case. You need to know whether it uses a standard ATX Form Factor if you want to try changing the motherboard. If it does not, and certainly Dell did not used to use a standard form factor, then you cannot change the MB apart from buying if direct from Dell if they sell them separately.

Re tuning. I suggest you visit PC Pitstop http://www.pcpitstop.com/ and carry out a free on-line tune-up check on your PC to see if that identifies any performance issues you may have.

Tomcat