Does CSS have the capability to do something along these lines: If the window size =< 800px, then blabla Else if the window size => etc.
No, CSS is a presentation language not a scripting languague. You'll need to use JScript or some other scripting language
For IE with CSS Expressions it's possible (with JSCRIPT), and many use that since max-width is not supported in IE6. http://www.svendtofte.com/code/max_width_in_ie/
I'm not exactly sure what you are trying to do with your expressions, but I find that most of the time I can get good functionality to work using the min-width or max-width CSS Properties. These properties don't work in IE 6 though. What are you trying to do? There may be a different way to do it than to use CSS Expressions (which only work in IE) or min-width, max-width (which don't work in IE 6), that would be compatible with everything.