It depends on which x6 you're over clocking.
I have a Phenom II x6 1065t, and it is stock 2.9ghz, and i've got it stable at 3.785ghz @ 1.5 volts (plus some other tweaking)
Honestly most multicore cpus running faster than 3.5 ghz will be very comparable. But once you breach that 4ghz barrier you'll be leaving all other CPUs behind.
My goal is to purchase a new CPU (as a back up and see how much abuse the 1065t can take. Here's hoping for 4ghz stable.