DD3 question/tri channed or dual? 2 types of mem.

[MagicUK]

Ive currently got Corsair (TR3X6G1600C8D) 6GB (3x2GB) DDR3 and I've just bought another 6gb of the same stuff 2nd hand so running 12gb with my i7 950, I've got the asus saber tooth x58 board and the stuff above with be running tri channel 3x2gb. Now round my dads and his bought some memory that's not compat with his ddr2 board that's his doing nothing and is £60 ocuk. The memory is corsair vengeance 2x4gb CMZ8GX3M2A1866C9 which I see is 1866 ram. Just wondering is it better to use the orig 12gb or the new 8gb stuff and buy another 8gb so 16gb in total. Obv in a performance point. Either way just wondering what peoples opinions are. On my phone so hope it makes sence