2 x 6950 will be a fairly small upgrade over 2 x 5870. The 6950's scale much better than the 5870's but the difference is small enough that its hard to recommend. It's a great combo though and probably your best bet if you can't wait. I think Sapphire has a nice reference 6950 with the dual-bios switch.
But I would totally wait a couple weeks for the 7970 to become available. Try one of those (they'll be expensive) and get a 2nd later if you like. All indications are the 7970 is going to be a LOT faster than the 6970 (and it better be considering its price!).