The Uncharted series is fantastic. Both games are tons of fun, albeit quite short (I beat the first one in nine hours and the second in twelve), but the story and gameplay make up for the short length in spades. Both are definitely must-own games. In HD, the gorgous cinematic environments really shine and look incredible.
Warhawk is also an excellent game. Addictive and fun hectic online multiplayer action with lots of explosions. What more could you ask for? Get it! For $40, you can get a copy that comes with a decent Bluetooth headset, making it even better of a deal. Looks just beautiful in HD, as well.
Mirror's Edge is probably the best $20 game I've ever bought. Also quite short, but to me, it was more than made up for by the fun I had playing through it. It's a good challenge, and with multiple ways through most of the levels, you get a good brain excersize in determining the best way to get to where you're supposed to be.
MGS4, from what I've heard, is utterly amazing, but if you haven't played the other games in the series, you'll be totally lost. As for KZ2, personally, I didn't much care for it; I thought the story mode was boring, and online, it's really the kind of game that's best played with friends, and since my friends never played with me even though they were the ones who pressured me into buying it, it ended up being a waste of money for me.
Anyways, some perspective from a big PS3 fan. Hopefully this'll help you on your way!