[jQuery] getting DOM values

Hi
BjQ (before jQuery) I can do this:
document.getElementById('foo').clientWidth
to get a DOM value that Firebug tells me is set to something other
than 'undefined'.
I can't figure out how to the do same in AjQ (after jQuery). I've
tried the following:
$('#foo').attr('clientWidth')
but keep getting undefined.
Am I trying the wrong syntax? If so, what syntax will achieve the
results I'm after?