Depends a lot on your requirements. Seeing you want to go with dual GPUs implies that you want to run today's games in high settings with perfectly smooth framerates. Doubt you can do the same 2 years down the road.
You could also consider the option of getting a 2nd GPU later on. Just start with 6870, 6950 and get a similar one later when prices have dropped (and when you need a bit of boost).