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.