Motherboard manufacturers commonly use phrases like SLI Support, SLI Compatible or SLI Certified on product packaging. SLI (Scan Line Interleaving/Scalable Link Interface) is a multi-GPU solution developed by NVIDIA. Motherboards that are compatible with Nvidia SLI utilize an Nvidia MCP (Media and Communications Processor). These motherboards are marked "Nvidia SLI Ready." Nvidia SLI also requires two or more matching GeForce PCI-Express video cards which are "Nvida SLI Ready."
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 approximately 256MB and another 256MB will be occupied after you install a second PCI-Express video card.
good summary Duncan Jones. i have plenty of ram so it isn't a problem. Nvidia recommends PCI-Express 2.0 x16 but I have just one PCI-Ex16 slot and that is in use by the Geforce 315.
so i will have to use my two spare PCI-e slots that are not x16 if I wanted dual gpu, but nvidia does not recommend this. Ok for this reason i am not going to have dual video cards but instead do a single upgrade on my Geforce 315 and replace it. thanks