72 dpi is the best resolution the human eye will be able to resolve from most monitors, so any larger dpi setting is wasted for posting on the web. A lot of people have 17" or larger monitors set to 1024 X 768 resolution, but there are always a few who haven't. I usually restrict the width of my images to 800px and try and limit height to 600px to minimize scrolling that online viewers would have to do (providing of course, that anyone actually views any of my stuff chuckle)
What program(s) are you using? Photoshop has the "save for the web" option that gives you a fair degree of control over minimising your file sizes for upload, where you can balance optimised/smaller file sizes against the inevitable jpg artifacting and loss of detail that occurs with a great deal of image compression. I hope this helps...umm, what was the question again?