I'd say both upgrades are pretty useless, your system is already very strong, any upgrade to your 2 GTX 660s would be an honest waste of money since it's already an excellent setup. I guess you could upgrade to something like a GTX 770 SLI or a GTX 780 Ti and stomp games to the curb but at what cost?
A CPU upgrade would be a bit less useless, though you'll probably need a new motherboard so the cost will be ridiculous anyway, about 400-450 for a good i7 4770k + Asus/Asrock mobo.
If you want 120 FPS in Battlefield 4 at Ultra everything 1080p resolution, then go with a GTX 770 SLI or R9 280x CF, if you want 1440p and above then GTX 780 SLI or R9 290(x) CF. And even then you may experience periodic drops in FPS anyway because of your processor, especially in multiplayer