You can reduce the resolution, yes, which will make things appear larger. It doesn't look great, but, with poor vision, it would not be too bad.
Also, using the System Preferences...->Universal Access preference pane, you can, in the "Seeing" tab, enable zooming, which lets you use control keys to zoom in and out dynamically - so you can see what's near your mouse.
Also, of course, many apps support larger scales within them for fonts, images, etc.
And, finally, many of the desktop features (icons, dock, etc.) can be enlarged.
These features, when combined, would let you keep the native resolution but still see things well.
Also, FWIW, Leopard (OS X 10.5) is supposedly going to have far more integrated scaling features.