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Use Xfce. Really. It is often mislabeled as an "alternative" or "lightweight" DE. Both terms are technically correct, but that misses the point that Xfce is currently the most usable and comfortable Linux desktop.
If you like the vertical panel then try to get hold of Xfce 4.10. Ubuntu still ships 4.8, 4.10 is available from Xubuntu via ppa.
Why 4.10? The vertical panel works fine in 4.8, I've been using on Squeeze since it was released.
In 4.10 you can make the panel look a bit like in BeOS, which, to me at least, is the best use of the screen space. If you only need a toolbar with icons (like in OS X) then 4.8 will do it just fine.