1. Buy 2x GTX260 graphics cards (doesn't matter about mixed v-RAM, mixed manufacturer etc. so-long as it's the same GPU)
2. Install them to the motherboard in the PCI-Ex16-2.0 slots.
3. Get an SLi-bridge and well, bridge them together (look for a diagram on Google perhaps).
4. Install the drivers from the nVidia website and configure it in the nVidia control panel.