ASUS's GPU Tweak II is crap. Use MSI Afterburner for overclocking and managing temperatures with the fans (called a fan curve). You do not need to own an MSI video card for it to work. Plenty of tutorials out there for that. Nobody can give you a specific number because everyone's airflow inside the case is going to be different as well as indoor room temp. What works for one guy may not work for you, and vice versa. You can also try Googling your specific card to see if there's a hardware review of it out there. Most places like Tom's, Anandtech, Guru3D, and TechPowerUp do overclock tests on their GPU reviews. If you do find one to follow, keep in mind their settings may not work for you as stated.