Don't upgrade your Ubuntu to 8.10 if you want to use a dual head setup. There second screen will reguarly turn off for a second while there is an edid ddc scan shown in the Xorg.log. (The bug is not really fixed or fix not in main repository yet. Or I've found a different one. -> It's still not working!)
And the whole system will stutter during that time. That makes video playback very annoyingly slow and usage an external monitor impossible. I'll let you know when I find a fix.