If you are hitting 65 then you are going faster than my YZ-125 will haul my ass around. Like Yz250JdT250 said, I am usually trying to shift to 7th....
I also recommend that you don't hold it at top speed very long, you will fry the engine.
If top speed is important to you then you should probably get a bigger engine bike. You can improve the top speed by going up a tooth on the front sprocket or down a few teeth on the rear but then you may have difficulty climbing the steeper hills.
When I ride my 125 with my buddies (who have 250's) I can generally keep up through the technical stuff, the whoops and even kick their ass on some of the hills. But when we hit the fire/pole line roads I just have to watch their dust disappearing into the distance.
Rod