Definitely the GPU
For _only_ 150$ to upgrade with, however, options are somewhat limited. Even the 660ti still comes in ahead at around the 300$ mark. The rest of your machine is solid.
My suggestion: hold for more cash. Look for something around the 660ti level to match to the machine. (as a single gpu system, your real factor holding you back is the gpu)
My advice for futureproofing. 660ti, 3GB of ram, or better, along the curve. You can get by with 2GB atm, but the games are getting bigger and bigger. And it looks like you hold onto hardware for a while.
On the AMD side of the line, the rough equivalent is the 7950, give or take. An argument can be made for the 7870 GHz edition if you want to try and get by with 'only' 2GB of vram.
Good job on that 860 build... that was the sweet spot on the cpu for a while. (the amount of cache provided, the speed, etc... not the fastest on the block, but one step removed, and much more economical