I'd recommend Ratchet and Clank: Size Matters; it's short, but you can get it for $20 or less, and it's a great game. KZ: Liberation is great, but I see you're already gonna get that one.
If you're a Star Wars Battlefront freak like I am, I'd say get Renegade Squadron because I've had tons of fun with it, but I think that one is more of a tossup on whether or not you liked the others.
Also, if you like racing games (read: WipEout), I'd recommend WipEout Pulse. (Haven't tried any other racers on the PSP yet ^_^)








