If you're thinking against COD2 because you don't think it will last you that long then avoid Kameo like the plauge... most people I know with it can beat Kameo over a long weekend. Don't get me wrong it's a fantastic game, I've played through the demo and over a few friends houses and I'm considering buying it myself but i wouldn't be buying it if that was my only game for a few months.
You'll definitely want a game that has an enormous amount of replay value...
Halo 2, as far as I'm concerned has crap for replay value, the solo missions are boring and the game is easily beaten, multiplayer is fun for a short while but becomes annoying as hell very quickly with all the cheaters and 10 year old that think calling strangers the N word online is somehow fun. Not to mention with all the great new FPSs out now Halo 2 is getting less and less play and soon the only people you'll be playing against are bargain bin rumagers or 10 year olds who can't afford new games.
My suggestion to you:
Forget about Halo 2, Forget about Kameo
Pick up Quake 4, a decent length FPS, it comes with Quake 2 so you'll get a whole 2nd games worth of play out of it. Plus it's got good replay value seeing as it's got multiplayer online (not the best of the available FPSs but good none the less).
Then wait for Oblivion to come out and get that. As it will undoubtedly last you hours and hours and hours much like Morrowind.
If you're looking for the least amount of games to get you the most amount of replay value then that's my suggestion to you.
If you're looking for a good long racer then pickup PGR3, another game that will last you weeks and weeks easily.
If all else fails hock some of your other stuff...
