Checkout Biodom's post
https://bitcointalk.org/index.php?topic=1764803.msg17678108#msg17678108 on where to measure the voltage.
You can use one of these guys to measure power-draw

Search ebay/aliexpress for "usb voltmeter amperemeter/ammeter"
Edit: Forgot to mention, vcore of about 1.5V should get you close or slightly above 300Mhz but you'll need proper cooling, that's for sure. These guys get very hot.
Speaking of which, does anyone have a particular meter or seller they would recommend, with the case and screen similar the picture above?
I just looked and there are about 500 available on eBay running from about $0.01 to about $20. I don't mind paying a little extra for quality or a bell or a whistle, but that price range presents a lot of options.
EDIT: While I'm asking, any recommendations for a decent multimeter as well? Billions to choose from.
Thanks!
You know, I was out, really out, and then my dang nephew badgered me into learnding him this stuff, and so, here I am. Again. However, I am very glad that he is interested in something like this!