will an amd fx 8120 processor be fine with the nvidia gtx 660 ti?
Yes, and just to add, the performance differential in most games would not justify the expense of swapping out your motherboard and CPU for, say, an Intel cpu at this point. I'd hold on to your mobo/cpu and grab either the GTX660 or a Radeon 7870 card if you're gaming at 1920x1080 or less. The graphics card is by far the bigger constraint on your gaming!
If you're running at 2560x1600 or planning to get 3 monitors, I'd spring for the 7950 with 3Gb or memory. I've got a Gigabyte Windforce 7950 that I snagged for $300, and I've flashed it to 1000MHz (it has a dual bios switch) using the Gigabyte 7970 bios, so I don't have to use overclocking software. Runs beautifully, and with three monitors, the 3Gb of ram is a nice insurance policy. Now, it pretty consistently beats the GTX660Ti and even the GTX670 in many games, especially at the 5040x1050 resolution I'm using.