I have gone over and over again, back and forth regarding graphics cards, having the GTX 470 myself it kinda makes me side with the 660
Do you have any benchmark comparisons for all cards like this link. http://www.videocardbenchmark.net/video_lookup.php?gpu=Radeon+HD+7850
I was basing on performance for price, as initially you were after saving the pounds for the build. There is most likely a few on the net but I found this one which near enough matches up to what I said in that the 660 is slightly better but for performance and value then the 7850.
http://gpuboss.com/gpus/Radeon-HD-7850-vs-GeForce-GTX-660
Here is the performance chart from your original link too which bears the same, it all comes down to choice and as you have been using the GeForce then you have a good idea how it runs etc in real world.
http://www.videocardbenchmark.net/gpu_value.html
Re the CPU, can understand your logic behind it and perfectly understandable for the backup as such. Think you would be very unlucky though to have a GPU fail after three years (warranty) when it is just left at stock clocks etc, but as you say backup is handy.