What can I do so that the entire width of my website will show on many browsers/OSes?

First of all, if you're going to use jello, don't measure your widths in pixels. Rather, measure in percentages or in ems (but percentages are a bit easier to control).

But since you mention "entire width", I don't even think you need to be specifying widths for your top-level containers. Typically, just playing with the margin and padding values of your body element will keep things simple enough.

A good idea to remember in the future though is to test everything in as many browsers and screen resolutions as possible before publication. OSes don't matter much as rendering is really browser-specific such that pages working in Firefox in Linux will look the same in Firefox running in Windows or in Mac OSX.

PS: comment on above post. DO NOT TELL YOUR BOSS TO GET HIS RESOLUTION UP. It's his computer, he should be able to use it as he likes. You work should be the one that accommodates your visitors, not the other way around. :)

You'll have to adjust the design for your audience... not everyone has a widescreen monitor, and not everyone has the browser window open full size all the time. If you insist on having a wide page, be prepared to annoy or lose viewers. The laptop I'm typing on right now has a 1024x768 screen, I can't make it any bigger.

No, (well yes, but unfortunately not very easily.) Just wondering how wide you are using? If it's more than 1000 pixels then I would suggest making it narrower. If not, then less than 5% of people are still using a 800x600 resolution so it will only affect less than 5% of visitors. Tell your boss to turn his resolution up.

It is tough to tell whats going on without seeing your code...your best bet would be to visit a web design forum like over at http://www.graphicdesignforum.com/ in their web design section - plenty of people can help over there. :)