Smashing Magazine has tweeted a very informative link on how to minimize web page load times. It lists some great tools which analyse load times.
Archived entries for requests
Basically, say you have one rollover button and want to have 3 states for it, normal, hover and active. You create 1 image file with each of these images in the one file. Then you specify (in CSS) what co-ordinates of the image to go to based on the mouse state.
So you have one image file loaded when the page is accessed, only one http request and less resource usage.
Here's the smashing magazine article explaining everything you need to know.