OK, I have done an update. 800x600 users should not see a horizontal scrollbar anymore, and the (middle) content part is a bit bigger in relation to the sidebars. Still small, I know, but that's the way this theme is going to be. I will do a fluid full-screen 3-column layout next.
@solarpunk: the flexibility of WP1.5 means I do not need to use just CSS to get 3 columns. I'm using separate divs, that also get called as separate PHP includes (I hope separate files will make things clearer for those who want to customize the code).
@BuffyVS: I personally prefer 2-column layouts as well, but there are already some good looking themes for WP1.5 that use 2 columns (like Kubrick and Manji), and because I see people asking for 3 columns, I thought I'd make such a theme.
When this theme is (more or less) ready I will release it under the GPL, so everyone can use it. This (among other things) is my way of saying thank you to the WordPress community who give me such a nice tool, that I also use professionally.
I'd still like to hear ideas about what to put into those sidebars...