Upgrade from gtx 460 to gtx 660 ti

That would be a big jump going from a GTX 460 to a GTX 660 TI, however I'd be concerned about how much your CPU is bottle necking you. Would depend a lot on how CPU to GPU intensive the game is.

Upgrade your CPU/MOBO to something like a Phenom II or intel equivalent before upgrading the video card imo.

Yes you'll get high fps increase but with a weak processor like you won't be able to play your games with much pleasure.You can experience significant fps drops,choppy gameplay,stuttering and the true power of the 660 ti will be constantly held back.
So you must upgrade your current CPU,it is so obsolete for today's games that I can't even suggest a lower tier GPU that can do the job for you without being bottlenecked.