The GTX660 is slightly stronger than a 7850, but not so much that you could tell a difference without a synthetic benchmark.
If you liked the performance with the 7850, you will like the GTX660 just as well.
Most games are limited by the graphics card, and not so much by the cpu.
Your X4 was a very good chip in it's time, and is still decent today. But, the fact that you saw a fps drop compared to an i5 indicates that your games may be somewhat limited by your cpu.
If a GTX660 is a big enough jump over your current card, I say go for it.
If it is only a smallish jump then you might rethink.