Well using the SLI option you will have the common SLI problems: you will need a bigger PSU,you will probably experience micro stuttering and some games might not even be able to use SLI.
No do not have anything yet. This is for my first build.
For an SLi rig I would first look at the resolution that it would be pushing in order to start contemplating which cards to use, this isn't to say that SLi is a must but I prefer to run two mid range cards that will give the same or similar performance of the flagship of that range and having done that for the past three rigs it hasn't worked too badly. It has always cost less in the initial outlay and by the time more performance has been required better cards have come out and I then have had two cards to use elsewhere or sell on.