You may be better off buying the best single card GPU you can with your budget now, and then you'll have the ability to sli in the future(15-18 months) and get a bump in performance. I'd go with a gtx 580, or if you can afford it a 670.
umm id say budget
cant afford around 400$ right now
and i can get 2 470 for around 300$
and i already got a 460GTX but i think its faulty too much freeze and problems with graphics so i want to get something better
but i cant afford too much
already got the new mother board and new CPU
which by itself cost a load