Well, if you already have a start with a set of Canon EF lenses than it really makes alot of sense to go with the Canon DSLR system. The XSI is the newest entry level DSLR, but the just replaced XTi, which I have is an excellent camera with lots of great features. do not be fooled by the term "entry level" either. These cameras are loaded with good stuff and great Image Quality and the XTi can be had for under or about 500.00 now, I would think.
As for pricing the E-300, search through Craigs list and Ebay to see what they are getting for them