One of the testers on Tom's ( Crashman ) started seeing problems running at over 1.4v. Personally I am at 1.35v with temps in the 60s with Prime 95. I could squeeze a few more hundred megahertz out of this chip I'm sure but 4.5 is plenty fast for me.
Just as a counterpoint, I've been running at 1.504v for a 24/7 overclock since I put my machine together during the last week of January. I made sure to keep my temps below 75ºC when I had "big air" cooling, and a recent water cooling kit purchase keeps the peak Prime95 temps no higher than 61ºC.
I run Einstein@Home so even when I'm not actively using my computer, it keeps all four CPU cores loaded as well as using CUDA to keep my GTX 570 loaded too. I tend to reboot once a week or so, unless there's a specific need to do so sooner. I don't power down unless there's a storm or something, so the machine's been running pretty much constantly since January.
I wouldn't suggest this to everyone, and I've probably been lucky ... but it is possible to run long-term at 1.5v CPU voltage.
For most users who don't need to go extreme like I do, I recommend keeping the CPU voltage at or below 1.4v for an everyday overclock. The peak Prime95 core temps should be kept below 75ºC.
Just as a counterpoint, I've been running at 1.504v for a 24/7 overclock since I put my machine together during the last week of January.
I asked this question in another thread...
When you set your voltage to 1.504v, and then load up your CPUs, how do you keep the voltage from increasing to higher levels? When I set my CPU voltage to 1.4v for a 4.8GHz OC, and then run something that loads up the cores, my Vcore will spike to 1.56v. It goes back down when the run is complete.
When it does that, it means that you're not setting the voltage high enough. The board will give it extra "emergency" voltage in that case. That's what I found on my board at least. If I increased the voltage in the BIOS, the load voltage actually decreased. When I set it at 1.4v in the BIOS, the load voltage would be 1.6v because the board was feeding it too much emergency voltage. Thankfully I noticed right away. When I set it at 1.45v, the load voltage was 1.55v. At 1.500v in the BIOS (and +0.004 Additional Turbo Voltage), it doesn't increase beyond 1.504v at load. Unfortunately, I don't know if all boards do this, or just my ASRock P67 Pro3 board.
Your response depends on what is more important...
If the max voltage is more important, then you will need to decrease your CPU speed until it draws no more than 1.4v at load.
If the CPU speed is more important, then you will need to increase the CPU voltage until the BIOS voltage and load voltage match.