This is something I have been trying to figure out for a few months now.
Here is my build:
AMD Vishera 8350 (OC'd to 4.7ghz currently) with a Corsair H100i Liquid Cooling system
2x GTX 680 4gb SLI
ASUS Crosshair V Formula-Z motherboard
16gb RAM (I think it's at 1566mhz right now)
Windows 7 on an SSD (Plus a couple games)
850 Watt PSU
Corsair 650D case
Whenever I play games, my FPS seems to fluctuate wildly at times. In BF4 I see my FPS go anywhere from 40-90 depending on what's going on and what map I'm on. In LoL my FPS goes anywhere from 30-120 depending on the mode I'm playing. In Skyrim my FPS is almost a static 60, with occasional drops into the 30's depending on what I'm looking at. I know my setup isn't absolute top-of-the-line or anything, but I honestly feel like I should be getting more out of it. Overall it's never very static.
I have people tell me that they have static FPS in these games and they don't have as good hardware as I have. I just can't figure out why I'm being sucked down with this build. I know with BF4, I'd get better FPS if I upgraded to Windows 8, which I'll probably do in the next couple of months. I've considered upgrading to the new GTX 780Ti's or something, but I've read that 2 680's should beat out a single 780Ti.