The difference that you will see with SLI depends on games. Some run it better than others. For some games it does a big difference, for others it is worse than having only one card. But for most you'll see an interesting FPS boost.
Air cooling is good for entry-level, and mid-range overclocking.
With liquid cooling you can get one step higher.
With water cooling, everything will depend on your budget. Can range from $150 to... some crazy astronomic prices.