Why does my (pretty good) PC not run any game well?

My PC, which is pretty good, can't run Counter-Strike Global offensive higher than 30 FPS, Battlefield 3 runs okay-ish, Even Minecraft runs at about 100 FPS max, while I feel this should be a lot higher. But it is just having a hard time with graphics. This surprises me, because the specs are as followed:

AMD FX-8320 8-core CPU (3.50 GHz/8MB Cache/AM3+)
Gigabyte 970A-DS3P AM3+ Motherboard
2 x8GB KINGSTON Hyper-X Fury Dual-DDR3 1600 MHz
3GB AMD Radeon R9 280 Graphics Card
Corsair H55 Cooler

It is properly cooled; it has 4 fans, plenty of power.
I'm, however, running 3 monitors, 2 of which on a separate, old graphics card, an nVidia Quatro.

AMD says they have no new drivers available, so old drivers are not the case.

Thanks in advance, please let me know if you want any more details.

Added (1). I have a 120GB SSD C-drive, of which half is available, and a 500GB HDD, 60% available.

RazzBudTazz 23.06.2015

Make sure you have good storage space on it, stuff doesn't run very well when i have too much stuff on my mac

Addy 23.06.2015

Get Nvidia 960 for $200 and your games will run at almost max settings.

Ant 23.06.2015

Interesting. Try this -- disconnect the 2nd and 3rd monitors and the old graphics card -- then see what frame rates you get. That R9 280 card should be able to support multiple monitors without the old gpu being in the box. And you'll still be able to dual or triple box -- you have enough processor cores to dedicate one to each 'toon.

some mobos [i didn't check specs on yours] throttle back the bandwidth to the primary gpu when there's a second gpu in the box… Which makes even the primary gpu run slow

Spock (rhp 23.06.2015