Buy that card, you'll be just fine and happy with it.
I have the Sapphire reference design card and play SWTOR, WOW, Divinity 2 and a poop load of Steam games all on high or ultra res with no issues.
I can also run three monitors off it just fine, even though I currently just run one 27" but when we were testing the GPUs for Lion the 6870 dis just fine.