How to properly float two columns side by side with css

This is one of those things I learned a long time ago and never thought much about if I was actually doing it the right way.

Let's say we have a structure like so:

<div id="wrapper">
  <div id="sideBar"></div>
  <div id="mainContent"></div>
</div>

Let's also say that wrapper has a width of 600px.

Should I float sideBar left, and mainContent right, or should I float them both left?

Additionally, if I set a fixed width for sideBar how can I make mainContent fill up the rest of the space similar to how a table works? If I set mainContent to display:inline-block and width:100% it moves down onto the next line :/

Note: In my specific scenario I do not want to use a table.