adraincs in Folding@Malaysia thread can achieve 4GHz, undervolted to 1.15V (stock 1.2v).
Since you have already achieve 5GHz (was it a stable 5GHz with 2hr prime95? If so definately a big achievement), now why not challenge yrself:
1) to see how much you can undervolt to achieve 4Ghz stable for 2hr prime95 (underclocking = "green"

).
2) how high GHz you can achieve with stock 1.2v
adraincs also mentioned that as long as you stick to 1.2v, it consumes the same watt regardless of how high GHz you run at. Don't disable stepping/turbo and those power saving parameters, so that when it is idle, it still goes down to 1.6GHz and around 1v (+- depand on the particular chip).
Buy a "kill-a-watt" meter to measure electricity usage in different configurations for best performance/watt would be very cool now a day.