Couple of things missed. First off buying a new cpu (one of the newest lines sandy/ivy or fx) does not mean they will not work in 5 years (unless you happen to fry yours for some odd reason). And they will still game. My athlon 64 x2 4400+ toledo still plays the latest games although i've personally never tried to go above the lowest settings (eye candy does not matter to me the fun of a game does).
Second off 4 cores will not be obsolete in 5 years. Why not? Since the release of dual cores few games (until recently) utilized them. And almost no games now make full use of quad cores.
Thirdly game companies try to make their products as widely accessible as possible. Meaning they try to keep the system specs low. This is why my old amd dual core is still able to play almost any game (i've not met one yet i could not play but i mainly play mmorpgs or rpgs).
And yes intel does give better performance in games. And you can find countless benchmarks and such that show case the fps gains of an intel chip when gaming. And of course intel will have the best cpu as amd is no longer trying to compete with intel at the high end market. The biggest baddest amd chip was designed to compete with an i5 2500.
If fps gains are your top priority then intel is the way to go.
http://www.tomshardware.com/reviews/gaming-cpu-review-overclock,3106-3.html
Just to show that at the $125 value the 4170 is a comparable choice for a gaming cpu as a i3 3220. And amd has 2 more cpu releases this year 1 in October and 1 in December. So some of the other fx chips may drop in price soon making them an even better value. And who knows amd may actually have a diamond in the rough in one of their new cpu releases.
Would i recommend an amd cpu over an intel for gaming specifically? No but there are some amd cpu's that are a pretty good value and game well (note i am not saying better). And some of these chips do well in other types of builds where gaming is not the sole focus. But in gaming the gpu has more to do with over all performance gains over the cpu (provided there is no bottle neck).