John --
From a strictly technical side, you can set the Priority number for each
Microsoft Project plan by clicking Project - Project Information and setting
the Priority value. That's the easy answer.
The more difficult answer is how to address the business and political
concerns within your company to prioritize all 300 projects in your
portfolio. I believe that would be better handled by your company's
executives and project stakeholders, since they are the ones funding the
portfolio. Hope this helps.
--
Dale A. Howard
Enterprise Project Trainer/Consultant
Denver, Colorado
http://www.msprojectexperts.com
"We wrote the book on Project Server"