Well Unless you just want to spend the extra 500.....the GTX 285*2 will be enough for a while. Kepler is coming soon, why not just wait and see when they are doing. the low level kepler cards could be equal performer's to the mid range 500 series
The 6950 would be a worthy upgrade by itself (non crossfire) I would keep the 285 and buy one 6950, if you like physx games you could try the physx cracks they have to use a nvidia card with radeon as a primary gpu. that way you get better performance from this without losing your physx. If the one 6950 isn't enough, you could always buy another down the road.
I should also mention Crossfire/sli wouldn't be worth it on that old cpu in your sig. It will be bottlenecked pretty severe, one card or one card plus one for physx however you should be fine.