There are two versions of SLI. SLI x8 and SLI x16. For optimal results, SLI requires a motherboard with two or more PCI-Express slots with matching PCI-Express speeds/lanes. Nvidia recommends PCI-Express 2.0 x16 slots (16GB/s). It is recommended that you also use a Windows 64-bit operating system with 6GB or greater of RAM since a PCI-Express video cards will occupy approximatley 256MB and another 256MB will be occupied after you install a second PCI-Express video card.
When manufactured, a motherboard with SLI Support has these features enabled by default or SLI Support is a free BIOS update from the motherboard manufacturer. SLI Support is clearly indicated on product litereature and motherboard specifications. To purchase a new motherboard with SLI Support, please consult the Nvidia SLI Compatibility List.
To identify your motherboard part number, I would recommend running CPUZ. Within CPUZ, click on the Mainboard tab and it will identify the motherboard part number.
With the motherboard part number, you can then visit the website for your motherboard manufacturer which will identify if your motherboard presently supports Nvidia SLI or if there is a free BIOS update available which enables Nvidia SLI Support.