ok, so, here's what I typed before, or something like that.:
First of all, what you are going to need is a Soldering iron, with a fine tip, and a very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very, very steady hand.
Secondly, you're going to want to have an alternative cooling solution set up. Might I suggest something like found here:
http://homepage.mac.com/paul74/MDDG4CoolingMod.htm
or here:
http://www.verax.de/verax.php?templa...44&lang=en
or (if you want to water cool) here:
http://homepage.mac.com/rgazaway/Wat...rCooledG4.html
With that out of the way, lets start explaining the (over)clocking process, to the best of my knowledge. The processor is "clocked" by a set of resistors on the processor daughtercard. These regulate something, which in turn regulates what the actual processor's frequency is at.
Because you want something in plain english, I assume you are referring to a site like this:
http://homepage.mac.com/paul74/Multipliers.htm
That's actually quite simple to interpret, because on your processor daughtercard, you will see several spaces (or soldered connections) with the following:
PLL_EXT
PLL_0
PLL_1
PLL_2
PLL_3
So, now we can go about interpreting what all the 0's and 1's mean.
On your 1.25GHz processor, you are set up with a solder or resistor bridge on every one of those PLL's, except PLL_3.
If you want to go to, say, 1.42GHz, according to the multiplier table you will need to "open" (read: remove resistor or solder joint) from PLL_1 and PLL_2, and then close PLL_3.
What keeps you from going to , say, 1.587GHz you ask? Well, your ability to cool it. For a processor running that fast, I can guarantee you'll need the water-cooler.
LEt me know if you have any more questions. This is the best I can do without having a processor daughtercard right in front of me.
Good luck!
-Kris