all of above u mentioned r done. the thing is one of our web based software is on linux and the one i m talking about is on windows. the linux software whoever made it.. contain slicing from photoshop as far as mine one is purely css based and optimized images, as well as proper tag ending and vice versa but still its slow. does it have anything to do with linux vs. windows hosting?
Use repeat-x or -y when it's possible if You have background images, and resize them.
I would also recommend using background color instead of images if it's possible.
You can also use programm like HTML Tidy to shrink your website code.