I have the Phenom X6 1055T - I would say that it depends on what you do with the machine. If you just game, the X4 965 BE overclocked is really the better choice. Virtually no games use more than 4 cores. Heck, most games really only use 1-2. So the X4 BE would get you more ghz (than either the athlon or the x6) and more cache (than your current chip) and be better for gaming than the X6 or the Athlon by a lot.
If you multitask a lot though or do other processing (video transcoding mainly), then 6 cores is the way to go. I do a lot of VM work and video transcoding, so I wouldn't trade my 6 cores for anything less.