Well that is only a 66mhz overclock which isn't much. Keep raising the HTT 5mhz at a time, I usually will run a SuperPi 1M test just to test quick stability. Then once the 1M test wont finish you know its unstable and you need to raise the voltage. I would probably go to somthing like 1.45v then start raising the HTT agian. You can keep doing this but I wouldn't go past 1.55-1.6v for everyday use. That's if you have good air cooling.
Are you using stock cooling? If you are I would really watch the temps, also you probably shouldn't raise the voltage too much. Like not past 1.45v
