I've had my 3500 running at 2.8GHz on stock air, 1.45 Volts... However, my motherboard and RAM are crap, literally... I set my RAM to run at 100, so when I overclocked, it ran at about 120-odd, I can't remember the exact number...
I also had to change my AGP rate to 4x, my video card would lock up if I kept it at 8x...
I also changed the HT to about 600MHz, instead of normal 1GHz.