Okay, in that case, take the deal. A 6950 for a 5850 is a good deal.
The 6950 is better than the 5870 in a number of ways. The 5870 was a power guzzler, and reviews pointed to the 5850 as the better of the bunch. Also, the new architecture is better suited for current games, and you can only expect improvements through subsequent driver upgrades.
You'll most likely get a 2GB model, so that'll probably be useful at very high resolutions with AA. Additionally, you can try performing an unlock which makes the 6950 run a bit more like the 6970, although personally I'll save that for when the card runs out of warranty (if ever).
It's a good deal. The new card should have warranty too, right?