I've got similar specs to your system and have just gone from a GTX560 to GTX670 in SLI. Only reason I went SLI is because I'm gaming at 1440p and I'd like to play at ultra settings. I think one card for 1080p will be more than powerful enough. The Asus 4GB that you linked is the same ones I have and they are great. Run very cool and quiet and pack a grunt. What more could you ask for?
Correct. One card is enough for 1080p.
I use two because of 3d mainly.
