I appear to have made an 'oops' by ordering a set of 3 corsair XMS3 1600mhz modules for my new i5 760 setup. On a GA-P55A-UD3 system, I understand that by having 3 memory modules it'll all run at single channel rather than duel. So I'm left with a single DIMM, and it doesn't look like you can buy that RAM singularly (to pair it with the one I already have) to give 8gb to my system.
No. Check the specs carefully. You should be able to find a similar ram stick, but try the three first if your board manual says it's possible. The performance difference between single and dual channel isn't that great.