I have pretty bad eye sight and i use a 27' iMac.
I have ALWAYS used Windows and adjusted the DPI settings but found the DPI settings to be really really annoying as it would always conflict with some app or program and really throw the screen or window off at times.
With my iMac, i set the screen res to a decent size, when i need to enlarge things a simple CNTRL + and it enlarges. Macs ZOOM feature is also very easy to use, A simple press of a key and swipe of the mouse and i am zoomed in.
I set all my fonts larger in Mail/Safari and other programs that allow me to do so. So far it has been WAYYYY better then Windows and its DPI settings.
I also LOVE that fact that the new iPhone 4 or iOS ( whichever added this new feature ) has large font increase DPI options now for text, web, mail, etc....Now i can actually enlarge my text messages!
Play around in Universal Access settings and you might find some helpful settings.