Crossfire is putting two video cards into one computer.
Personally I don't think over-clocking is worth it. It shortens the lifespan of the product (either CPU or GPU) because they run hotter. Sure, you get 10% or 15% more performance, but so what? The only over-clock I've ever done is on my i7-4770K Haswell CPU, and even then just a little bit. I do a lot of video re-encodes of recorded TV shows with Handbrake, it produces a file about 50 - 60% the size of the original file, and it can take up to 90 minutes to encode, so by giving my CPU a little boost I can cut 10-15 minutes off the encodes. What's nice about my Haswell 4770K is that the overclock doesn't effect it most of the time, it just boosts the top frequency, so that when...