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.
I have poor vision as well, lowering the resolution causes things to look more blurry, text is not as sharp. Yea, things are bigger, but it looks like I have even more blurry vision. I just have my face two inches in front of my screen.
I decreased the resolution on a MacBook and it wasn't bad. My iMac has been set back since I bought it and it looks fine to me. It's certainly better than not being able to read the screen (besides, if your vision is bad you are used to things being slightly blurry).
Remember, even at the highest resolution, you can increase the system font size and the size of text in most browsers, word processors and drawing programs. Personally, I would get new glasses before using universal access (but it is there if you want it).