Oki here is the deal. I am using a laptop - now and then connected to an external monitor. KDE3 is able to detect dual-head setup (BigDesktop or MergeFB) and manage windows when lvds and ctr1 are connected and I it doesn't use xinerama for that.
As far as I understand xinerama extension should be used only on xorg servers with multiple graphical cards and monitors.
Then again KDE4 (4.0.1) does not recognize BigDesktop setup and I am forced to use either crt1 or lvds but never both at the same time. I believe dual-head support is not planned by 4.1...

The true question is, if I am right so far, is it possible to use xinerama with fglrx on dual-head setup so that KDE4 could manage windows across both screens.

How much overhead would that create? (Xinerama vs BigDesktop vs MergeFB)

Regards