Fixed vs Fluid vs Elastic, and basic setup Options

[barrykins]

Well. I've been going back in forth in deciding on how to design websites overall. Fluid is amazing, yes. But however, it can pose problems to people that have cinema display Macs. Right? I develop on a macbook pro so my 1440x900 resolution would be different then that of someone one a windows.

I guess Im looking for input on what you design in and kind of the basic set up to let everyone be able to see.
As such I use the 960 grid for laying out my wireframes but I'm confused about one thing. How big do you exacly make the Background image so a screen with a larger size may be able to see it?

This is what I currently use to set up the main containers and bg, am I correct? Centering is a issue with em because I don't exacly understand the em measurements. Any input would be nice. Im majoring in graphic design so all web design is kind of self taught.

CODE

html, body{
background:url(..) no-repeat;
background-position:center;
height: 100%;
margin:0;
padding:0;
width:1000px;
}

#container {
position:relative;
min-height: 100%;
width:960px;
}

#main {
position:relative;
min-height: 100%;