Both Nvidia's SLI and AMD's Crossfire allow you to combine/add a second, third or sometimes even a fourth similar generation graphics card (or add in more GPUs) to the one you already have in your PC.

Explain that Multi-GPU mode you talk about... Okay I tend to get a little repetitive with this question, but honestly... is there anyone who visits this website that doesn't know what SLI & Crossfire ...