Get a Xbox
I wouldn't recommend a console to any serious gamer. That being said I would personally go for a PS3 as developers have reportedly barely scratched its abilities.
OP, I think you can pretty much last four years before these SB's can't hold their own in newer games, but you'll definitely want to upgrade sooner, probably buy a next-gen CPU in a couple years. You're SB won't be "outdated" per se but you'll want to replace it.
The thing is CPU's are pretty much hitting a thermal limit right now, and so the current trend is to parallelise applications to take advantage of multiple cores, which, fortunately, scale linearly regardless of how many you use (provided your application is designed to use them well). So while before, your 2.2GHz processor could sort of work through a game requiring 3GHz or better, in the future you'll have your 4-core CPU trying to keep up in a game that takes advantage of 12 cores. And if you do the math, that is a lot harsher on the CPU. In my opinion in a distant future you're going to buy generic, brandless CPU cores that physically cannot be any faster and add them to your computer. That's right, you're not gonna see 50GHz clock rates - it's a thermal impossibility.
Well, that's putting quantum computing aside of course