iMac and external projector - low resolution issue

I try to use an external projector on an mid 2011 iMac running Snow Leopard. The iMac is connected to the projector using a VGA adapter. DVI is out of the question as the beamer supports VGA only.

When I configure the screen resolution using display preferences I always see "Usable resolution 800x600" regardless of the configured resolution. 800x600 is not really useful for me as I need to display detailed information.

The projector is definitely able to display higher resolutions using a Windows Box or a MacBook Pro on either Snow Leopard or Lion.

I even switched to another projector model, this worked for a while. At one point in time things changed and I am limited to 800x600 again.

Any ideas or help would be greatly appreciated