Setting keyboard shortcut to switch to an application
I would highly recommend installing QuickSilver which is an excellent application launcher:


http://qsapp.com/


In QuickSilver you can also setup keyboard shortcuts for various actions (called Triggers)  including launching an application.

If you click here


http://qsapp.com/about.php


and then browse through the feature description until the eighth pane, you will learn about QuickSilver Triggers.

You could also have a quick look at the manual which is found here:


http://mysite.verizon.net/hmelman/Quicksilver.pdf


Triggers are described starting on page 34.

Hope this helps.