Beginners Guide to Dual Graphics Cards?

So I installed a


"EVGA GeForce GTX 570 HD 1280 MB GDDR5 Dual PCI Express 2.0 DVI/HDMI/DisplayPort SLI Re... " in my computer last year, and last week my SO swapped his Nvidia for something better at bitcoin mining, so now I have his "1x NVIDIA GeForce GTX 570 1.2GB (Includes PhysX Technology)" sitting on my desk. The two don't look alike, and mine has that "HD" in the name, and is about 6 months younger than his.


Is there some sort of baby guide for when it is a good idea to use a second graphics card (I was considering trying to MB rift next month) and whether these would work well together, and what that "HD" in the name of my card even means?

http://isboxer.com/wiki/GPU_Management has some of what you want.