No you won't have to set it back to stock to install and run the 660ti. However, if you are experiencing crashes it could be that your CPU overclock isn't actually stable. Or it could be something else. But it definitely is NOT that your CPU isn't powerful enough to handle the GPU or game settings. When you install your 660ti, try it at stock CPU settings and then try it with a slight CPU overclock (make sure its stable). If you don't notice a difference in graphics performance then there isn't a need to OC the CPU is there? My guess is that you won't see much difference in your gaming experience with a CPU overclock.