Oblivion- that game is so long it's ridiculous. On a single play with 1 character, you'll probably spend over 120+ hours getting all of the levels, completing all of the quests, and finding all of the locations (there are hundreds). If you actually explore each location- you're looking at a 200+ hour game

Killzone 2. I see you have many CoDs already, so it is clear that you like FPS, and Killzone 2 is the second best that I have played this gen, so the multiplayer should keep you busy, but it still won't last 2 months, no go will

Oblivion.

Never has a game been released with so much value for your buck.

looking at your list i would recommend you
Burnout Paradise,
Saints Row 2,
Midninght Club LA,
Far Cry 2,
Skate 2,
Orange Box(though i heard ps3 version have many issues),
basically every good sandbox game will do the job,
looking at your game collection i doubt you would like Sacred 2 after all - it have strong old-pc-rpgs kind of feel

Killzone 2 has amazing multiplayer that will last you a few months.
Oblivion will last a month or two.
LittleBigPlanet, I still play a lot (and I got it day one)

Any one of those games will last you a long while.

Vegas 2's levelling system will keep you going for ages.
Burnout: Paradise will take a long time, too, and it's very fun.

Honestly, the first game I thought of is Oblivion. It's a no-brainer.

thanks everyone, and yeah, I'm thinking I'll go for oblivion, it looks much more upbeat than fallout 3

Bunrout Paradise... tough I played it on PC every singel Burnout game since Takedown took away months of my life ;) Especially Legends :D Now that was a portable masterpiece :D

I would recommend Oblivion GOTY or Valkeria Chronicles in tandem with another cheap action oriented game so you can mix it up.

For me the games with the highest replay value have been:

Street Fighter 4
Oblivion
Little Big Planet
Resistance 2

If you can score them for under $30 try out:
Uncharted
DMC4
MGS4
Motorstorm Pacific Rift
Wipeout HD
Folklore

You can also get many PS2 gems for under 10 bucks these days
God of War 1/2
FF 10/12
MGS3
etc. etc.