I run a Radeon HD 7870 with a 100 MHz factory OC (which is a little bit faster than 5850 Crossfire, or a GTX 660 in most games) with an i5 760 and I really haven't encountered any CPU bottlenecking, CPU usage typically will cap out at around 80% for multithreaded games with my setup. You might hit a slight bottleneck if you went up to a GTX 670/680 or Radeon HD 7970, though you would still see a decent performance gain. You do have a P55 mobo, so you can try overclocking your CPU if you do hit a bottleneck, just be sure to replace the stock cooler before you do so. Most games are single or dual threaded, and turbo boost will boost one or two cores up to 3GHz or so when only one or two cores are loaded, so the Lynnfield CPUs still perform well in gaming, even with the lower stock clocks.
If you already have a 5850 Crossfire, you may want to aim higher than the 660 though, as that will be more of a sidegrade than a true upgrade. The only reason to move to a 660 or other midrange card from a 5850 Crossfire would be if you were having lots of driver problems related to Crossfire, or the 1GB framebuffer on each card is becoming too small for your liking. If you are willing to spend more money to get a higher end card like the GTX 670/680 or the Radeon HD 7970, you may want to add an aftermarket cooler for your CPU, just in case you do need to overclock.