Hi. GTX 680 SLI means that you will get two GTX 680s. Two GPUs are much faster than a single GPU, but only in well-optimized games / tasks. Otherwise, the performance increase is not that big. It's more of a luck if you get the double performance out of SLI setup rather than 0% fps increase.
Honestly, i don't recommend you do that, here are a few reasons:
1. Your power supply is too low. One GTX680 requires up to 200watts of power, SLI means two GTX680, which means only 400watts for an SLI setup without other components - you only have 450watt maximum.
2. I highly doubt that your motherboard supports SLI configurations. It does however support CrossFire, but SLI is a bit different and i didn't find it anywhere written in manufacturer's webpages that they do support SLI, so it shouldn't work.