A stock E6600 is still fine for most casual gamers, and my E8400 @ 3.6Ghz still absolutely crushes every game I toss at it. Great chip.
I hear people talking about how great Dragon Age is with quad-cores, but I run it at 1920x1080 with max settings, vsync on, and 8xAA and it's buttery smooth throughout. Not sure the exact fps but it "feels" like 60fps constant. Again, vsync is enabled so nothing above 60fps is of any consequence.
And yet, anyone buying a new cpu today would do well to grab an i5. There's no reason not to. My next cpu will absolutely be quad core (duh!) but I'm in no extreme rush since my E8400 is still performing so insanely well in games and everything else I use the computer for.