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For gaming the Phenom X2 550 3.1GHz is the best choice i'd say. Newest fab process and highest clockrate will make single-threaded games run faster than AMD's triplecore of 2.8GHz.
Since most games are single-threaded, they use only one cpu core; so frequency is most important here. One extra core is nice to do additional windows stuff, for example part of the sound system during gaming can run on the other core.
RS780 is 780G chipset, while the newer 785G chipset offers more performance. Also the southbridge should not be older than SB750. This will help in overclocking the Phenoms.
Since most games are single-threaded, they use only one cpu core; so frequency is most important here. One extra core is nice to do additional windows stuff, for example part of the sound system during gaming can run on the other core.
RS780 is 780G chipset, while the newer 785G chipset offers more performance. Also the southbridge should not be older than SB750. This will help in overclocking the Phenoms.