help - problem with screen resolutions & external tv / force disable?

I'm running a 1,8 GHz G5 iMac and have an external tv set connected to my system via the apple mini-vga/s-video adapter. As soon as I connect the adapter, my screen resolution on the primary display switches to 800x600, and even though I can change the res back to 1680x1050, the OS X interface stays at the small size in the center of the screen.

Is there a way to have my primary display run its native resolution full-size and configure the external tv res independently, so I can still work with my iMac when the second display is connected? Mind you, the mirror mode is fine, I'm not looking for the Spanning Doctor... (or would that help me with this issue?)

If this can't be done, can I at least disable the second display via software somehow? Currently, I have to manually disconnect the adapter every time to be able to use my first display...