If you have Elder Scrolls: Oblivion then it has to be either Fallout 3 (amazin game - get GOTY edition for the DLC) or Dragon Age (another brilliant game).

Them two will keep you going for hundreds of hours.

Fallout 3 GOTY edition
Modern Warfare 2
Bad Company 2 (coming soon)

And there's plenty of old games you could get for

You dont have Uncharted so i suggest Uncharted 1 & 2.

What have you got yourself?

uncharted 1 n 2 , resistance 2 , killzone 2 , infamous , metal gear solid 4 , motor storm pacific rift ,dead space , modern warfare and grans theft auto are all good cheap games

I would recommend checking out a few PSN games:

"Supersonic Acrobat Rocket Powered Battle Cars"
Can be best described as soccer with cars. The game is easy to learn, but extremely hard to master. Single-Player gets boring soon, but if you're playing online or splitscreen multiplayer, you can easily have dozens of hours of fun with this one.
Check out the free demo, it's very extensive and you will instantly know if it's your type of game.

"Mushroom Wars"
Nice little strategy game, has a free demo too.

"Pixeljunk Shooter"
Another great game that got lots of excellent ratings.