I'll toss in a recommendation for Ace Combat 4. I didn't really care for AC3 on the PS1, but I really like AC4. You can beat the main story mode in about 3-5 hours, so if you're the kind of player who likes to blow through a game, see the ending, then move on to the next game it's probably just a rental for you. But if you plan to unlock all the planes and weapons, play through the higher difficulty levels, and check out the other game modes, it's going to give you at least 15-20 hours of game time. It's part of the Greatest Hits line, so you should be able to find it for $20 or less.

Of course, if you're looking for retro games, don't pass up Activision Anthology.