If you max out settings of games like GTA IV or even Crysis 2, CPUs like Q9650 @ 4GHz or AMD X6 @ 3 GHz, they are really pathetic, really really pathetic. And The Q9xxx series became outdated about 2 years back.
I wasn't able to get the smoothness I need from any less than a Core i5/i7 Quad since 2009 Half 2.
And FPS is only part of the deal, smoothness, snappiness, butterness and no stuttering are many other things which FPS doesn't tell.
The fact that you are not even aware of this much shows your level of awareness in technology. And of course you have zero experience based on the statements you made.
Enter 2014 and Core i9s and i7s won't be playable at max settings, it could well be in mid 2012, but 2014 is the very max when even the fastest of today's CPUs will need to be upgraded, irrepective of the overclock.
And I can't think of a single game today which doesn't use at least 4 cores, for practical purposes!
And for your info, a comparison btw 2600k and 9550
Compare Intel Core i7-2600K (Sandy Bridge 4c/8t), Core 2 Quad Q9550 2.80GHz Processor
Gaming is the area where CPUs show the least difference and even there:
Left 4 Dead 2
260 FPS on 2600k
170 FPS on 9550
Now that is more than 50% difference. And please don't be idiotic enough to say that both 170 and 260 FPS are enough because in future apps including many of the currently released games, it will be 17 FPS and 26 FPS respectively, and currently, it is probably 34 FPS and 52 FPS respectively which is a HUGE difference.
And on an average, in most CPU intensive games, a Core i7 2600k = 2 x 9550 and for other things it could be 2-10x times as fast!