I am looking to upgrade from my 9800gt as I just upgraded the rest of my system.
Here are my specs:
Asrock Z68 Extreme 4 MoBo
4GB Super Talent Cl7+8GB Corsair Vengeance Cl9 running both at 1333Mhz
2TB Caviar Green HDD with 64GB Caching SSD
I want to either get a gtx560 ti and sell my old card
Well the two cards you are considering are very similar in performance with the HD6950 being slightly faster at reference speeds and the GTX 560 Ti pulling ahead if you take overclocking into account. If you may want to add a second card/switch to a higher resolution or 3 monitor setup then perhaps go for the 2gb model of the HD6950 as the extra memory will help there.
Since you have upgraded the rest of your system why are you wanting to keep the 9800gt , upgrade to the two 560 Ti's and you can get better resolution and higher fps and they can handle the physics as well. Since there are very few games that support physics you will benefit more from the two 560's then getting a 6950 and keeping the 9800.
I agree about keeping the 9800GT for PhysX being fairly pointless but 2 GTX 560 Ti's is quite expensive. Nice if you have the money but a single card, especially if given a nice high overclock, is still quite capable and a huge upgrade over the 9800GT.