NightD: Resolution IS how the Mac controls the geometry. There is no other way. If the Mac is putting out 1024 x 768 resolution, then the geometry is 1024 x 768.
Whether this displays correctly on your TV depends on the capabilities of the TV. Most TVs are not made to handle computer signals, and are doing well to achieve more than 500 lines of vertical resolution. If it is a widescreen LCD HD ready TV, then it could handle up to 1280 x 768 (the first HD resolution is 1280 x 720). Some widescreen TVs have controls for resizing standard aspect material by either stretching it, or by putting black borders left and right.
Bottom line: You're going to have to deal with the TV, not with the Mac. Your results may vary depending on the TV.
Some corrections:
Two monitors displaying different content is called spanning.
The S-Video connector is a single round connector that has (4 I think) pins within the shell plus a plastic key. It is coincidentally the same connection that Apple used for Apple Desktop Bus (ADB) keyboards and mice.