I have an MSI GTX 670. The first thing you should do is turn on the OSD in Afterburner and hotkey it so that you can turn it on and off in game. Feel free to play with the settings, but with my card, I got the best results from leaving everything at default except the "power" slider. Turn the power slider to maximum to allow the card to draw more power and overclock itself more.
When using afterburner, I found that overclocking manually would only send the card into a lower processing state faster and cause it to throttle faster. Logically, it makes no sense. I do not understand why it does that, still, I would get higher clock speeds under load on the default settings, no kidding. Heck, on the default settings, it will already...