Phenom II X3 owns the X4 620 in gaming with the extra L3 cache. Add that to a far higher overclock potential and you clearly get more for your $$ than that extra core on the 620 IMO. It's not a sure thing unlocking a chip either, people are already posting threads with chips that won't unlock so its best not to bank on it happening.
My vote goes to investing cash in cache and make up for the lost core via overclock.
thumbs up on this recommendation. if you game, PII x3 720 BE is the defacto cpu to get for it. you will appreciate the 6MB L3 cache and most games show triple thread currently so this will be excellent on core count as well as cache needs for a game. you will also get a shot at unlocking it to a quad if you lucky. also it can do about 3,7ghz oc.
btw, I just got the $115 combo with athlon x2 550, no luck unlocking it at all and oc is limited to below 3,5 due to the low quality of the chip well they charge less you get less. so stay away from this one.