1) You can expect a 60%-ish boost in performance typically, sometimes much better, sometimes much worse depending on the game.
2) Adding another card doesn't double the available VRAM. Since both cards are working in tandem each using their own memory all of the textures load onto each card seperately. 1.2GB is still plenty for most games at 1080p resolutions, but some performance hit may be seen in games like BF3 at 1600p, or when using multiple monitors.
3) You'd crush either card. They're probably more on par with GTX 560 SLI.
You might want to wait a week or two to see what kepler brings to the table.
SLI will give you very nice benchmarks on games that are suitable for sli, but you may not get much on games that do not play well.
If just one game is important to you, do some research.
Another possible issue is your 750w psu. It may be marginal for two GTX570 cards.
If you are gaming on a single monitor @1920 x 1200, then something like a 7970 or the possible GTX680 may be a better choice.
Either will run on a psu more like a 500w unit.
It seems like nvidia is trying to clear out inventory of high end cards, so you might find some very good prices on gtx570.
On the other hand, it suggests that the new kepler cards will be very price/performance competitive and an early purchase of a second gtx570 may bring you buyer's remorse.
Go for it. You can expect a pretty good boost over a single card though it depends on the game. I wouldn't worry about the VRAM too much unless you plan on going with more than one monitor. For one monitor you should be fine for some time to come.