VRAM doesn't yet matter on 1920x1080. I'd go with 570s, though it's very, very close. I tend to favor Nvidia, but then I've got one in my system. They'll trade benchmarks; each is better in about half of current games. Definitely the 1.2gb version.
If you go nvidia, do yourself a favor and go with EVGA. Best customer service ive ever dealt with. Also the stepup program on their reference cards is nice too if you decide you want a more powerful card in the first 90 days. Waiting on my 7970 to arrive today, its on the truck and out for delivery, so no green here altho I wish EVGA made Radeon cards
It's weird that those benchmarks show the 6870 crossfire setup better then the 6970. Because my buddy has crossfired 6870s with an I5-2500k and he gets worse frame rates then I do with my 6970 with an OC'd FX-4100.
I would expect 6870s to get a better measured framerate, but they might have micro-stuttering problems that could mess up your perceived rate.
There was no perception of better frame rate. With fraps on and both of us on ultra and 1920 x 1080 I was getting much higher frame rates. He gets great frame rates at high, but at ultra I run away with my 6970.