If you want a game to last you for a couple of months, I would suggest the Crysis series. 1st of all, you can max it. 2nd of all, there are TONS of things to do on the island. Explore/mess around with/approach things. 3rd, there are TONS of MODS and levels to download for extra gameplay. Overall, it is a GREAT game.
Farcry 2 was fun, but only for a bit. It is VERY repetitive. It's almost ALL go there, talk to him, ok now go blow it up. that's it. The traveling is a little "dry."
Fallout 3 expansion won't last you for long. and you know why... it's only an expansion lol..
Crysis gets my vote.