Hawk is a great card and brings tremendous value over a stock 6950 anyday. With the advent of newer drivers, sli scaling is also almost twice now and as good as amd.
In general, 6950 gpu core is a tad faster than 560-ti core. So the 560-ti needs to run at faster clocks to catch up to a 6950. Although at fullhd, there's no difference.
Msi 560-ti hawk is a heavily overclocked card and has a fantastic cooler. This is why its recommended over a reference 6950. But its match is a 6950 Twin frozr III and will beat the
hawk anyday and will run cooler too despite being overclocked.
Only in multigpu setups, 560-ti will lose to a 6950cf because of its extra framebuffer and in higher resolutions, the difference will be more. Hawk sli will bridge the gap a bit but won't beat a 6950cf even at stock. Talking about the 2gb here strictly.
But the 560-ti sli will allow you to play physx games like batman arkham city to be released this october.
@op
Consider a single 6970 now. Go for msi 6970 lightning and you'll be covered with all your needs. Buy from india as you won't get warranty if buying from abroad.