You might have searched for lots of resources on transparent PNG with Google or any other top search engines. 

But a recent better solution is not yet top on google.  But tops in CSS community.

http://www.alistapart.com/stories/pngopacity/

Here you will find best script solution as of today.  Remeber PNG is for Web 2.0.  So, learn it today how to handle it for some genius(?!) browsers.