overclocked 1800+ heat and stability questions

[Ohmsford]

Well, after alot of playing around with my Bios settings, I got it up to 2170 (155 x 14), and so far so good. I've been running Prime95 for almost an hour, and so far so good, but its up to 57 degrees Celsius (I've got a volcano 9 humming along in there). Now i'm wondering, is 57 a little high? Its been up to 60 before and I never had a problem with anything, that was with the settings at 166 * 12, and it was stable. Also i'm wondering how long should I let Prime95 run to test stablility? I've always just let it run for about an hour, and if the computer didn't die, well it seemed stable enough.