Good overclocking cores are AXIA, AYIA, and AYHJA. I know that AXIA is slightly better than AYIA, which is slightly better the AVIA, but I'm not sure yet on the new AYHJA core. Some weeks do better than others. AXIA Week 9 was a pretty sweet week IIRC.
BTW, these numbers can be found on the second line of code on the core of the chip.
Nick