alternating classes with widgets

I am in the process of developing (my first custom theme) for a client. One of the things I need to be able to do is to have the class surrounding a widget alternate dyanamically so that if there are 2 widgets the first one would have <div class=”widget1″> and the second would be <div class=”widget2″>

Then the next widget would go back to the first class and repeat – preferably 3 classes switching would be great, but 2 will work.

Can this be done and if so how?