4,5 ghz = great
4,6 ghz = great
4,7 ghz= also great
4,8 ghz = now, this is great, work on getting this one stable.. You prob wont get much higher stable 24/7..
4,9 ghz = Starts to go downhill (instability/heat), still kinda stable, but not like the 4,7 ghz..
5,0 ghz = not 24/7 solid stable (even if it might run for a week or so without crash, it prob will crash sooner or later. No real use for this OC, in 24/7, the gain is small, and the negative starting to appear now.....now stretching the physics of the cpu, heat/Voltage/instability..
5,1 ghz = Easy to OC, no boot issues (if profile is good), instability, heat(voltage. Not for 24/7.. will crash in time also.. Less stable than 5,0 ghz.
5,2 ghz = its up to the cpu specimen, and the overclockers skills.. massive heat, voltage, may be boot issues, You must be slightly nuts to make it somewhat stable (benchmarking)..
5,3 ghz = Like the 5,2 ghz, almost all common sense for GENTLE overclocking is gone, this clock may include very high voltage settings/massive heat, boot issues, BSOD, you name it.. bonus OC..
5,4 ghz =Please dont...
5,0 ghz and above = is for benchmarking, if you ask me..