Further OC could also depend on your MB, CPU stepping, CPU and overall cooling. I'm keeping mine at 4.8GHz @>1,4v 24/7 but under Mugen3 cooler. Had it at 5.GHz but 4.8 looks like a point of diminishing returns as it needed at least 1,5v and cooling was right at the edge.
As for how much it helps at gaming, it also depends on particular game and how much of CPU it uses.