The GTX570 is from Galaxy; how is there support?
Probably pretty bad. Small company that produces pretty much the cheapest GPUs in each class. And the card itself only has one fan, it will be noisier than 6950 2GB with two fans. (GTX570 consumes more power to begin with although it's 10-15% faster).
I'm guessing the 6950 is a much better overclocker than the 570?
Both can be decent overclockers, it depends on the particular model. In this case I'd expect XFX or Sapphire 6950 to OC better than a single-fan Galaxy 570. None of these are built to be overclocked per se - if you want big overclocks then look into MSI Twin Frozr III PE/OC.
Also does VRAM really matter?
I'd say yes, it does matter. Everytime I say that though there are people who disagree, but there are also those who back me up.
VRAM use depends a lot on the game you're playing and the graphics settings you're using. So how much VRAM you will be able to utilize does depend on how fast your GPU is, i.e. how high-end settings it can take advantage of. When the VRAM of a card gets maxed out, the GPU will start to use system RAM instead. System RAM is many times slower than GDDR5 and that will have a noticeable impact on framerates.
I use a 560 Ti 1GB which is slower than 6950 2GB, and I've encountered the VRAM limit in games including Crysis 2, Rage, CIV 5 (DX11) to name a few. Skyrim with custom 2K textures would also probably push it well over 1GB. In some instances, maxing out VRAM has made the game crash for me, but I don't know if that's common.