SLI is sticking 2 identical Nvidia graphics cards into your computer so that they run together. Ususally two cheaper cards linked together would give you slightly better performance per dollar than a more expensive card, but you have to deal with increased power needs, larger power supply requirements, more heat and a bit of image quality loss due to flickering shadows etc.
Your power supply is fine, but its 500 W, so you wont be able to do SLI or Crossfire(same as SLI but for AMD cards).
So if you dont know what SLI is, I suggest you stick to a single card config. This being the case, you dont need the extra pciE slot on your motherboard, and can get a micro-ATX board with fewer pciE slots for slightly cheaper ($20-30)