I am not a gamer and i am in the market for a new computer based on the I5 prcessor. It will be mainly used for video editing and conversion between different formats. Is SLI of any benefit to me?
SLI is Nvidia's process of pairing two graphics cards together to gain a performance increase over one card, and it is similar to AMD's Crossfire. It does not double the performance; performance increase will scale differently based on the card you are using and the program that is running (some programs/games dont support it and you would only get the benefit of one card anyway whereas other programs can see nearly double the performance.)
In your question you asked whether SLI support is needed on the motherboard, and the answer is yes IF you wish to use two graphics cards. (Note if you use AMD cards you need to ensure the board is Crossfire compatible as well). However, in answer to your question on whether it will provide a benefit to you, it may not give a benefit that is comparable to the amount you spend on a second card (meaning the performance you get vs the price you pay.) and therefore may not be required. It can be a good habit though, to get a SLI capable motherboard in case you intend to upgrade later on, and most ATX sized motherboards support it.
Video encoding/conversion is normally more dependent on the CPU, as this is what does the work. Therefore, spending more on another graphics card will be of little value. I am not too sure about video editing however, and i cannot give you an answer to that part, a second graphics card may help or it may not, but as i will go into further one good one should be fine.
For normal tasks, some users find the on-borad graphics on the newer Ivy Bridge enough for basic tasks and they do not even require a dedicated card. AMD's new series of APU's are better at this function: They provide a processor and a good graphics card in one package. However, the CPU on AMD's APU's is weaker than that of an intel i5 processor, so it comes down to whether you want better CPU (intel) or graphics (AMD) performance if you choose the route of not getting a dedicated graphics card.
However, for your situation i would recommend staying with the i5 you have picked and buying one dedicated card, it should be more than enough (this will depend on how much you spend on a graphics card, a cheap one might not cut it etc). Like I said, some programs don't play nice with SLI or Crossfire, and investing in one good card is often the advice given on these forums, as well as the fact you will have lower total power consumption.