|The graphics cards are where you may need additional power. Without knowing what cards might be in use we can't advise you. I would assume you plan to use top of the line cards or there would be no advantage to SLI over a single powerful card.|
If you intend to build a gaming machine out of that rig you should have gone with a larger case that can hold a standard ATX PSU.
Some hight end graphics cards can draw about 20A@12V each. 2 of them are not going to run on any available mATX PSU. Then you will also need to deal with heat. Small cases are more prone to overheating. Poor circulation and undersized rear case fan.
You may want to rethink your entire plan. Also, IMO, SLI/crossfire are a waste. Only advantage is when you are running 2 of the best cards on the market. Otherwise you can do as well with only 1 of the best cards. Double the cost, double the power requirements, twice the heat.