There are a couple of FAQ's in the FAQ section which give some very good pointers.
For AMD 64 chips (I don't have exp with Intel, but some parts are the same)
1. Clock Speed. This is the base speed in which a reference pulse is generated and sent to the different components.
2. Multiplier. This is the number of times that the clock speed is multiplied, as logic circuits will only work on the up or down or both edge of a clock cycle.
CPU Multiplier. This multiplied by the clock speed gives your CPU core speed.
HTT link. This is basically the link to your northbridge (from memory), you shouldn't really take this above the max your board is rated at. The multiplier usually goes from 1x to 5x in 0.5 steps, but you shouldn't use the .5 steps. A lower the max speed doesn't really slow things down.
Normal ranges are 1800 or 2000MHz (2 * Clock speed * HTT Link multi)
RAM Divider, for A64 this is a bit different. Some boards show a MHz speed, some show a fraction . This will be your RAM speed at the default clock. BUT if your board shows it in MHz, you will still need to calculate it from your clock rate e.g. clock = 225MHz RAM set at 333MHz (DDR)
RAM speed = 2 * (275 * (333/400)) = 457MHz, which is a 57MHz overclock.
PCI/PCIe speed, this is done as a divider of the clock speed. You should also be able to lock these. As they easily become unstable when overclocked, its best to lock the speeds at default.
Basically you need to balance the clock speed with the CPU miltiplier and the RAM divider, whilst making sure you don't go above the max HTT link speed and the default PCI/PCIe speeds.
To find the max CPU speed, lower the HTT to ~3 and RAM to ~1/2 then slowly increase the clock speed while maintaining the highest/default CPU multiplier.
To find the max RAM speed, lower HTT to ~3 RAM at 1/1, and CPU multi to ~1/2 max/default, and then slowly increase the clock speed.
You may need to increase the core and mem voltages slightly to make them stable, and keeping them cool is a must, an aftermarket cooler is advised for both the CPU and the northbridge. Also you require a decent
PSU BEFORE doing any of this. I don't know the max safe voltages, but someone here should be able to help with that.
Basically you want to find the max of each RAM, and CPU speeds, then balance 1 against the other, as you wont be able to have the max of each. Also you motherboard will have a max clock speed it can handle.
Running a synthetic benchmark will help find the sweet spot for your gear, as it's different for every machine, even with the same models.