ati or nvidia for this set up?

[H3RBSKI]

so im looking at building a new gaming pc, after asking about on the forum ive settled for this mobo/cpu/ram combo.
now all i need to decide is nvidia or at?
Core i5-2500K 3.30GHz
Asrock Z68 Extreme4 Gen3 Intel Z68
Corsair XMS3 8GB

ive been suggested to go for the Sapphire ATI Radeon HD 6950 TOXIC 2048MB
but ive always been more of an nvidia fan.
ive read that the mother board im going with is sli only so not sure if i want to limit myself to just one card in the future.
im thinking about going for a gtx 580 or a gtx 570 depending how much i have spare.
what are your opinions? should i go ati or stick with nvidia? want to try and make this pc future proof.

 

[H3RBSKI]

ah fair enough, theres not much info about that board on this site and all other sites i found suggested it was sli only.
that was the only thing stopping me from going ati