Honestly? There is no one single game any one person should own. Not because there aren't any great games, but because no two gamers have exactly the same tastes.

If I had to name just one game though i'd say Fallout 2. It's not a game, it's a life experience. Otherwise...

Action/Shooter: Half-Life 2

RPG: Fallout 2

Sports: Whatever the heck those people play that's "good"

RTS: Company of Heroes if your rig can handle it. This game... holy Heck... it's the most badass game i've ever played. I'm not going into detail but damn... why are you reading this and not playing it...?

MMO: CoH. I can't say WoW because it takes a certain type of person to actually stick with it long term. I say City of Heroes because it's simple, offers a little bit of everything, and because it's just fun. It's a good, well rounded MMO. Most likely wont stick with it long term, but you will keep coming back to it and have nothing but great memories.

6600 GT. Good card. 7800 GT. Great card. 8800 (whatever). WTF? No need. Expensive, something better will be out when DX10 becomes standard. It's like nuking an anthill because they wont die fast enough with napalm. I'd go for a 7800 personally. I have a GT and a GT OC, they've never failed me. There isn't a game out I can't totally max the settings and not lag in.