Firstly you need a special Kernel to do this so don't bother with the apps unless you have loaded a kernel supporting CCCR and VCORE change.
Secondly I will explain my personal distrust of overclocking....
Whilst the PXA270 is designed to run at Turbo clock speeds up to 624Mhz (you don't buy a 416Mhz PXA270 - it's the same component in all PXA270 devices) it doesn't necessarily mean that all peripheral devices on the board are also designed to cope with the higher clock rates. To achieve the high clock rates the PXA270 is programmed to actually generate clocking for memory and display etc. This could potentially be at the detriment to some components not designed for higher clock rates.
Secondly manufacturers design systems with power consumption and overall thermal characteristics factored into the devices. Firstly your battery life is going to suffer... no problem you may say but you generate more heat from the battery the faster you deplete it... also more heat is generated by the components running at the higher clock rates. If the design of the housing for the unit is not sufficient to accomodate heat dissipation for the intended duration of use then you may start to experience damage. - In other words exercise extreme caution when running in overclocked mode. Don't do it for too long. Don't play movies whilst overclocked and plugged into the mains because the battery charging ALSO generates heat. Don't use it unless you really have to.
Anyway, this is my personal opinion some of which is factual and some of which is speculative... I really don't know if the SL-C3000 is built to cope with a 624Mhz turbo mode... mind you I get twitchy over the amount of heat that my WLAN card generates and wont use that for too long either.
Since the PXA270 doesn't provide any thermal stats (and there's no hardware that I know of in the Zaurus to do this) the only way of quantifying the heat issue would be to feed thermal sensors into the device - which I'm not going to do.
There are two apps that will allow overclocking at present on the SL-C3000 - I thought I would take a look as one of the Cacko 3K alpha team was interested.
Here's tetsu's page (translated).
He lists both apps. qclockchange and batteryplus.
Please look at the table of CCCR/VCORE values for the applications before using qclockchange.
It is worth noting that batteryplus lists the CCCR values and not friendly names like 'Turbo Mode' etc. but the settings equate to....
02000210 = 416Mhz
02000214 = 520Mhz
02000218 = 624Mhz
Please note that the non default settings (02000214 and 02000218) actually reduce the clock rate of the display so you may see flicker.
Also note that tetsu shows the default VCORE setting as 10 for 416Mhz operation. If you select 02000210 on Battery Plus this gets reset to 9 meaning that the device uses 1.3v rather than the standard 1.35v - I'm a little curious about the stability issues associated with this.
Finally note that suspend/resume stability may be effected by having the device in an overclock/underclock state.
Full Cacko ROM preserves these settings and swaps them for default during suspend. Alpha1 team for SL-C3000 should note - I have NOT implemented this so exercise caution.