A 260 should do 660-700 with little trouble. They're good cards. If you can afford it get the 280.
Two 260s @ 660-700 in SLi would be very fast and they are at a nice price point now. 260s would also work with Tri-SLi. You can't run SLi in your mobo however Intel's X58 is coming up and it will be able to do SLi.
I'm a fan of single strong GPUs. An overclocked 260 will get the job done @ 1920x with filtering save in Crysis if you set all very high
. The new 260 with slightly more shaders is also an option. At the same clocks as the regular 260 it's about 1-3% faster. It's below the 280 in price. You might jump to the 280 instead depending on your budget.
One reason you may want to stick with nVidia is quite a few games will be coming out with PhysX support. You could use that old 8800GTS as a PhysX card in the mobo you currently own. I'll post some PhysX results below.
If I were you I'd probably kick that E6600 up to 3.0-3.2GHz and get the cheapest 260 or 280 you can find and OC it. BTW the E6600 I had back in the day would do 3.0-3.1GHz on stock voltage. With 1.52v it did 3.75GHz. If you are using the stock HSF stick to 3.0GHz.
Warmonger, results @ a 1440x res. Max settings. CPU is a Q6600 G-0 @ 3.51GHz.
GTX 260 doing PhysX @ 760|2600 1580 = 93FPS
Adding an 8800GT @ 700|2000 1750 = 123FPS
As above but 1920x-
Single GTX 260 doing PhysX = 71FPS
Adding an 8800GT @ 700|2000 1750 to do PhysX = 88FPS
I got roughly the same results with MKZ. CPU PhysX was only in the low 20s and GPU PhysX was 60+ FPS.
My results match up well with tests Firingsquad did-
Where PhysX may boost performance or add effects-
Running the various games that include PhysX support made me acutely aware of just how many things are effected by physics in gaming.
Below is an old post I did with my GTX 260 using card killer games @ 1920x like; Crysis, WiC and Lost Planet.
Below is a review that includes all the cards you mention in a ton of games-
Since the HD 4870 and GTX 260 are often very close I'd get the 260 or 280 to leave the door open for GPU PhysX support.