TBH, Im not sure bottnecking will be that much of a issue unless maybe on something like a 6970 or GTX 580.
Or, if running a Crossfire/SLI setup.
But for most Mid Range, single card steups, you should be fine.
Sure, there may be a few games thats CPU intensive, or really recommends a Quad Core, where you may see some .. but overall it shouldnt by that big of a issue.
Or at least I wouldn't think so.
Perhaps in the 6000 series the 6850 or 6870 would be a good choice?
The 6850 will require a 500W PSu though, and as you go up ... 6870, 6950 etc its going to require more.