With the limitation of 8x PCIE slots on that motherboard, your GPU choices are likewise going to be limited. Getting a GTX 660 or 670 or an RX 460 or 470 are going to give you the best bang for your money. Anything better is going to be bottlenecked by the motherboard. As for a CPU, if it's already got an X5660, X5670 or X5675, you're going to be good. If anything slower, an X5680 would be a good upgrade and worth the $70ish you'll spend. As far as ram goes, if you have 9GB or more you'll be good. But if you've only got 6GB, you'll want to get upto 9GB or 12GB. Remember that you have triple channel ram access and you'll want to stay in multiples of 3 to get the most out that system.
But for free, you've got a great system for some gaming! And it's got some headroom for upgrades.
EDIT; a PCIE USB 3.0 card would be a good and inexpensive upgrade. If you're interested, I can give you links to products that I use in my clients systems.