1. You need a good H/S fan to O/C, if you've got a retail version, ditch it and pick up an Alpha PAL35T for $30.
2. When you've got that, increase the core voltage to 1.85V or so and go for it. You can't change the multiplier on Intel CPUs, so up the FSB to 90 MHz. That'll get you to 855 MHz which should be reachable.
3. Then try for more, a couple of MHz at a time. Don't be afraid to go as high as 1.95V on the core setting (if your mobo lets you) as long as the Alpha keeps the temp below 40C max. It shouldn't idle higher than 30C with an Alpha on it.