Generally Alternate Frame Rendering 2 is usually best but this is not the same for all games so i usually just leave it on Nvidia Recommended.
You only need one SLI bridge for 2 cards.
Not all games benefit from SLI. Some even do worse. Games that use the source engine don't tend to benefit much. On my rig CSS does considerably worse with SLI enabled. However games such as Crysis or Farcry 2 benefit quite a lot.
EDIT: If you would like to overclock the cards then download a program called Msi Afterburner or a program called EVGA Precision. Both are very similar. These programs will enable you to raise the core, shader and memory clcoks, but of course with any overclocks, do this in moderation. 10Mhz - 20Mhz at a time (stress test between). Decrease the amount you overclock by as you climb higher and instability chances will rise too.
Hope I helped. Enjoy your stay at OCN!