Here is a living example of how I did it on my blog:
#wrapper {
width:900px;
}
= Overall width of the blog
.twothirdcol {
width:75%;
}
= blogposts width
.threecol {
width:25%;
}
= sidebar width (blogpost+sidebar = 100%)
.twocol {
width:49%;
}
= Width ofthe two colums in the blogpost (each)
In my case the .twocol only works with less than 50% (each) because I tweaked the margin around the images as well.
NO GUARANTEE that it will still look like when you visit my page, as I play around alot with the setting… 🙂
Cheers,
Alexander @ http://www.mouseclick.com