reuse, means use again. Meaning that upgrading to a more powerful CPU and motherboard would not require you to change the RAM, HDD, PSU, GPU, case, etc.
The CPU you are using is not really fast enough to push 2 high end GPUs, but it is the fastest processor you can get for your motherboard. So if you want to go faster then you need to get a new motherboard, and a new CPU that operates at a higher level than FM2 parts. However, your rig should be fine for most games. I am just saying that if you really do want to upgrade to a 2nd GPU, then you will want to upgrade some of your other parts first.