its not a 555 anymore....its a 560.
looking at AT bench on a x2 560 vs a X4 645...unless i'm doing heavily threaded apps, i'd rather take a x2 560. they both perform pretty similarly in single threaded apps, but the lack of L3 does seem to prevent it from taking the lead in anything. in fact, so as long as you are fine with a dualcore and willing to accept that it may only be a dualcore, you can always try to unl0ck it as a quadcore....
i wonder if the newer chip is any better; i'm pretty sure they are continuously trying to improve their proceses, so the new wafers on this new stepping (is it? what stepping are the 560s?) will hopefully have a lower defect count. means a higher chance (note: chance, not guaranteed) to have a chip that can unlock and be stable with little voltage increase