I also have an ATI card but its a 9800. I'm not sure how much of this will apply, but here it is anyway.

The span settings do not appear if you only have the drivers installed. You may need to download and install the "ATI Catalyst Control Center" which gives you a ton more options to mess around with.

For me (once I installed and ran the app) I went to [Standard View] tab, then [Display Manager] and selected [Wizard]. Following the directions on screen (a few [Next>] buttons) I came to an option allowing "Extended Desktop", "Clone", and "Horizontal Stretch" modes, with the latter being the one I picked.

Having messed with it for just a few minutes I still havent managed to get some custom shortcuts to display settings working correctly (e.g one version with a horizontal span, another with normal extended view) but I'm working on it.

If anyone has a neat way of doing so it would be cool to hear...