resolution detection with notification pop-up?

[Chadi]

I have 15% of my visitors using 800x600 resolution, which makes my forum home look cramped and horrible. Can someone please explain how I can setup a script that automatically detects a visitor using this exact resolution (or perhaps any below 1024x768) and popup a window warning, or static warning on the actual page (maybe header?)...with a custom message suggesting they increase their resolution?

Even browser / version detection also would be great too, along with javascript being disabled.

I would appreciate some insight on this. Thanks.