Don't worry about Furmark, that is a totally unrealistic scenario. Set a clock speed and then you can do a quick check with Heaven to see if it passes. If it does, then try some games. It'll take time to lock in the right clock speed. Also, try upping the core and memory a little bit each time together so it stays balanced. You don't really want a huge memory overclock right away, because that may be causing instability and limiting your core overclock. Something like +100/+300 should be pretty do-able.