Let me know how it turns out for you... I wouldn't fret either way, the difference between 1220 and 1280 on the core would be about 2-4 FPS difference on average anyway - nothing to shake a stick at but not a huge deal either. The 6xx series cards are more bottlenecked by their memory bus, so overclocking the memory will have more of an effect. Sometimes the increased voltage on the GPU will allow for a higher RAM OC as well. Your memory is running at 7388 MHz anyway, which is pretty good - mine is only stable to 7246 unless I have hardly any GPU OC at all.
Usually the higher you go on the core, the lower you will have to go on the memory - and the other way around... SeanPoe's GTX 670 OCing guide explains how to tradeoff GPU MHz for RAM MHz in the best possible ratio.