Microstutter can be VERY obvious at times. I dislike it a lot. It varies significantly between games, and graphics cards.
3xSLI generally produces very minimal noticeable stutter however that can get expensive.
SLI also does not add Video RAM. If each card has 1GB then the game can only use 1GB (the same data is on each card). Many games are starting to require more than 1GB.
The GTX670 with 2GB is a really great card. VRAM is sufficient for up to 2560x1600 and in some cases 5760x1080 though most games can't hit 30FPS on triple monitors with only a GTX670 or GTX680.
- Microstutter, and lack of VRAM are issues that need to be considered.
- The GTX670 2GB is (arguably) the best value and stock is, on average, 93% the value of a GTX680 stock.
- Get a non-reference cooler such as the Asus DirectCU II design.
- A single GTX670 will be far quieter than 2xGTX560Ti as well.
- sell the GTX560Ti while it still has value (value will go down significantly once GTX600 series is properly available and a $250 GTX660 or similar is launched)
The new Kepler GTX670/680 etc cards have many advantages that previous gen cards don't such as TXAA. I advise you to go to the NVidia site and read about Kepler. It's quite interesting. The UNREAL 4 engine is also being optimized for Kepler so far.