Guess i'll be needing a new monitor since my old one's busted....

I feel like getting a 20" monitor.... does anyone have anything to say about it?

Does playing games on a 20" monitor gives headaches or anything.... or maybe a 17" is better?

Also i have the choice of getting a NEW 17" monitor VS a second hand Phillips 20" monitor.... which one do you think is more worth it?

Any thoughts pls reply :P