Does anybody have any ideas on how I can move the left sidebar in?
I'm shooting for this:
http://dev.birr-group.com/concept.jpg

http://dev.birr-group.com/
ATA 3.7.3
WP 3.1.1

why not use a left AND left INNER sidebar, just put a single text widget in the LEFT and put your plugins in the left inner sidebar.

I tried that, but can't use percent in the width of the left sidebar.

Why cant you use % width? You could put the following in CSS Inserts

td#left {
width: 30%;
}

I would also try designing this with a fixed width which would make things a bit easier.