1.6A @ ~2.2 on a system nearing it's first birthday without having encountered any problems. Just do proper stability testing (memtest, prime95, etc.) if you overclock and keep an eye on temperatures. Not that stability testing a non-overclocked system isn't worth doing for peace of mind as well.
I wouldn't hesitate to try a couple extra hundred Mhz with the latest batch of P4s, not because you need extra performance at those levels, but because you can do it and do it safely.