SLI (Scalable Link Interface) is basically nVIDIA's version of linking 2 graphics cards together. What SLI does, is that it allows 2 SLI compatible cards to share the load, therefore making the graphics rendering way more efficient. 1 card does the top half and the other one does the bottom half. SLI is expensive and eats power, but no one ever said that all that power came cheap. The one without has just a single PCI-E slot while the SLI one has 2. If you can afford SLI then get the SLI board. Or you can just get the SLI board, buy one nVIDIA PCI-E card first, then get another one of the same type when you have the money.