Depends on the game and what resolution you play at.
The 560ti SLI is better and is quite fast and I have no issues with vram.
It all comes down to your budget and the rest of your system specs.
You may also want to consider getting a stronger single graphics card and going xfire/SLI later.
The Vram does not set how fast your card is it helps when you have more than one monitor or your resolution is more that 1920 x 1200. Unless you are doing what I just stated then I would get the 560 ti.
So here is a link the 6870 vs the 560 ti in SLI/CF. One more thing since the cards are so close if you are using higher graphics or more than one monitor then if it is cheaper then get the 6870. I hope this helps and good luck to you.
Would you consider the 670?
Single card with better performance, less heat, noise and power.
For approximately 400 bucks with performance close to a 680 its worth a mention.
But you will have to grab it quick as they will sell out quite fast.