I am rocking a 1.8ghz AMD Phenom quad. It performs as well as my friends 2.0ghz Intel quad. Like Smith said. Phenom II will be able to match intel's i7, the only difference is AMD is cheaper. Also more friendly to OC . Of course if you don't have top notch matching counterparts. I.E. good mobo, good gfx card, good ram...then your i7-i9 wont be worth much...Also make sure you are running Win 7 (64bit).
My motto on this is. If you have the money buy the best you can afford. I built mine as a budget gaming pc, So the only way it really impresses people is on the price it cost to build it. Haven't had a game slow me down yet, though I haven't been gaming much on it lately.