DDR2 and DDR3 are DRAM interface specification. DDR2 I/O x4, DDR3 I/O x8. So DDR3 can reach higher bus speeds.
Which is better for gaming? Well with the increased speeds comes increased latency that partially offsets the advantage of the increased speeds. DDR3 does have a slight performance advantage but that noticed mostly in benchmarks not so much real world.
But the MoBo normally dictates which can be used. If cost is similar get DDR3 if DDR2 MoBo is better value get that.
Something to consider with DDR3 is if the system is optimisied for it. I can say from experience, and talking with other OCn members, DDR3 isnt optimised for 775. However, with i7 memory controller on die, it does run nicely with 1366. So if you are asking if switching to a DDR3 775 board (guessing you are thinking of going with a 790i) would give you better gaming performance, from my experience I am going to say no. I have actually found the max OC on a DDR3 system to be significantly less then the DDR2 motherboard equivelant. I've run DDR2 X38 and X48 boards, and DDR3 X38 boards.
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