memoy for oc

[tek33]

hi i currently have 4x1 gb corsair c4d dominator ddr2 6400 memory running a q6600 g step with dual boot vista home pr 64 and win 7 home prem 64 .my question is i do a lot of gaming and i am wondering which is better to oc the 4x1gb or 2x2 gb ? is a evga 122-ck-nf67-t1 680 i sli some help please and if so what spec settings for mb and voltages ?