Overclocking Via Reference Clock
If you don't have access to an unlocked multiplier, things are a little more complex. You'll need to overclock by adjusting the Reference Clock. This can be a problem, because the Reference Clock also affects the speed of your Hypertransport link, your Northbridge, and your Memory. So, by adjusting your Reference Clock, you will also be adjusting the speed of all of these components. You're no longer relying simply on the potential of your processor.
Overclocking also becomes more complex because again, motherboard manufacturers don't have a standards regarding the designations they give to the options that allow overclocking. I've seen motherboards which call the Reference Clock the Front Side Bus, for example, although AMD Athlon X2 processors do not actually have a Front Side Bus. Again, having a motherboard manual to reference to is very handy, as it will help you decode what specific settings your motherboard's BIOS uses for overclocking.
Whatever your motherboard's BIOS, there are a few things you can expect to see. First off, most motherboards will have some sort of overclock master control that will need to be set to Manual or Disabled, instead of Normal or Turbo, so that you can tweak the overclockin options by hand. Secondly, the Reference Clock should also be listed as 200Mhz stock, so if you see a setting listed as 200Mhz, it is a candidate to be the Reference Clock. Northbridge and Hypertransport are both often reduced to NB and HT, respectively. Finally, Memory configuration is usually kept somewhat separate from the other multipliers - if you see an area that allows you to change DRAM timings and DRAM voltages, you're in the ballpark.
You'll now need to find the limit of your Reference Clock. To do this, lower all available multipliers between the Reference Clock and Northbridge, Hypertransport, and Memory to the lowest settings possible. Then begin bumping up the reference clock in 5mhz increments. This will increase the speed of your processor as well, because your processor's speed is determined by the Reference Clock multiplied by the CPU multiplier (the multiplier you didn't lower, and may have raised in the first step on this page). Keep increasing until you have instability when booting. Then back down. You've now found the limits of your Reference Clock in combination with your processor.
This is where things become complex. Because the Northbridge, Hypertransport link, and Memory all can be adjusted separately, you'll need to test each in combination with your maximum Reference Clock to see how far you can push your computer. Take it one at a time, increasing until you find instability with one multiplier, then back down a step on that multiplier and attempt to increase another. There is a lot of trial and error to be done here, but with some time you'll be able to find a comfortable overclock.