im working on my senior project in high school and found a great site to roam around for a processor. Great prices. First and foremost, set your requirements (number of A/D and/or D/A, speed of processor, clarity of converters,etc.). Here is the site:
www.futurlec.com
for futurlec, click the boards tab to see the great selection.
sparkfun is also good, but more expensive.
www.sparkfun.com