Okay, I'm now using the Dronix 0.4 ROM with a kernel that takes me to 691 Mhz. There are some kernels which go a bit higher but I'm afraid to push my luck.
It took me a while to find out how to do it so here are my instructions which I also posted in another forum:
0. Root the phone if you haven't done so already (information on this is easy to come by - I used a program called z4root. Rooting your phone voids your warranty. There's no way around it. You should also sync your contacts with Google so you get them back when this is over. You might want to make a list of your favourite apps because you will need to reinstall them after installing a new ROM and after each kernel upgrade.
1. Install ClockworkMod ROM Manager.
2. Open ClockworkMod ROM Manager and select the first option, "Flash ClockworkMod Recovery".
3. You are presented with a disorganised list of phones. Somewhere in there is Huwei Ideos. Find it and select it and then follow the on-screen prompts.
4. Download your desired Dronix version (Dronix 0.4 was the latest stable version at time of writing but if you're reading this from the future then that may have changed) and any cores you wish to install and place them on your SD card.
5. Open ClockworkMod ROM Manager and select 'Install ROM from SD card'
6. Browse to the location of your ROM and select it.
7. Select all the data-wiping options and follow the on-screen prompts. You phone will restart and will take a while (about 10 minutes) to complete the process.
8. If you're satisfied with and overclock limited to 648 MHz then just go into "No Frills CPU Controller" and set it up (lowest value as the lowest and highest value as the highest; set at boot). If you want to try higher overclocks then follow the process again from step 5 (you will need to install ClockworkMod ROM Manager again but don't need to repeat step 2). This time browse to the location of the overclocking kernel which you want to be able to use and follow the on-screen prompts as before.
9. Once satisfied, re-install all your apps. Synchronise with Google to get your contacts back. You will need to reinstall No Frills CPU Controller because it will be gone now. Set it to use the maximum core speed as maximum. Beware that you're doing this at your own risk. Overclocking a processor beyond what it can handle will result in instability and may brick the phone so you can't reverse the procedure.