I'm guessing that the option isn't there because the MSI Hawk has a custom PCB. Also, it won't throttle down when you're in the middle of a game. The drivers/whatever controls the core speed will make sure of that.
I have a 650W PSU. I don't even get close to using all 650W. Do you know what speedstep is for the Intel CPU's? It's the same general idea. If the N460 isn't in heavy use, it'll run at lower speeds to conserve power and to generate less heat. It is not a problem, but rather a nice little thing to keep the heat down. Sure, you might not be @ 100% load on your GPU even at 780MHz, but still, whatever counts, right? I wouldn't recommend updating the BIOS of the card. It's risky if you're not experienced at it. It's perfectly normal to have it downclock when not in use.