No-frills CPU control will enable you to overclock a rooted phone. A good way of beginning to determine your maximum stable speed is to download quadrant from the market and run benchmarks at progressively higher overclock speeds, until the phone freezes or reboots. But you should also test other tasks, like navigating through screens and menus, opening and closing applications, etc. Your max is the value beneath the speed that the phone freezes/reboots at.
My Monte Carlo doesn't like anything above 960 MHz. Any higher and I get almost immediate reboots.
*edit* After further testing, it looks like 940 is actually my max. It'll work at 960 for the most part, but I've had two reboots now meaning that it's unstable at that speed. Poll updated to reflect change.
Edited by TouchyAndalou, 18 October 2011 - 12:50 AM.