It's kinda hard to tell what's really 'rare' or 'valuable', since it so dependent on time and location. Suikoden 1 isgetting 'rare' now, but about 5 years ago I could've gotten everyone on this board a copy for 10-20$. Why? Because the PS2 was new, and no one cared about it, and the area's Fred Meyer store was clogged with the game. Then Suikoden 3 came out, people wanted the original, it got bought up, and since most people aren't selling it now, it's 'rare'.
And location- who here would believe me if I said Kingdom Hearts was a bit on the 'rare' side, and impossible to find non-GH? Proably no one, but I don't see it in every store, and I've never seen the non-GH version, save the 2 owned by my friends, and the 1 I bought from MightySlacker. So if anyone wanted to sell their original KH, this is the place to so it, you can get away with calling it 'rare' and charging 30 bucks, I bet.
Just buy games ya like and watch the used prices and availiblity- that'll tell ya if it's rare or valuable...
'Scarce' is the only definite, since it's tied right into print run.