Why can't I overclock a rooted Samsung Galaxy GT-I7500 beyond the default maximum 528MHz?

I have a rooted Samsung Galaxy GT-I7500 running stock Donut (Android 1.6). I tried a couple of overclocking apps such as SetCPU and CPU Master, but none of them let me actually "over"-clock the CPU, because I can't set the maximum to be more than the default 528 MHz. I have read about speeds of 624 and/or 710 MHz possible with either the Galaxo custom ROM or GAOSP, but I want to achieve those kinds of speeds with stock Donut. I've tried GAOSP b3 (Gingerbread) and Galaxo 1.6.3.4, but I prefer the feel of stock 1.6 (that's why I love my stock Nexus S running 2.3). Is it possible to set the CPU speed beyond 528 MHz in my rooted Samsung Galaxy GT-I7500? How?