I5 2500K or fx-8120 for gaming

all you had to do is type that into google, or look at a review of the 8120, any you would have your answer. If you have not heard, amd's FX cpu's are a flop, they are slower than their old Phonom II line of CPU's. The i5 2500k is a LOT faster, more power efficient and when ivy bridge cpu's come out you can drop one into your current socket 1555 motherboard for even more perfromance.