Here's a really brief guide:
You have a base speed setting, from which almost everything is set. This is often called the HTT (which is also used to refer to the two hypertransport links) or the FSB, though neither term is really right. FSB is probably the better term, though it isn't the same as the FSB on other x86 architectures. Asrock call it 'CPU frequency' in the BIOS.
Anyway, this is the key setting. Unless the BIOS has changed a lot (I'm running P1.20), you'll find it in the Advanced menu screen in the BIOS. Set the overclock mode to CPU/PCIE async. If you don't, any overclocking will overclock the PCI Express bus and it won't like that. The base speed setting, whatever you want to call it, is below.
You will also need to change some other settings, because they will be set relative to the base speed.
Firstly, the memory thing. By default, the memory runs at the base speed (EDIT: assuming you are using PC3200), so if you increase that in order to overclock your CPU, you will overlock your memory by the same proportion. That will limit your overclocking unless you have unusually fast memory. You will probably be able to o/c your CPU by 25% by keeping the same CPU clock multiplier and increasing the base speed to 250MHz, but it's very unlikely that your memory will be able to overclock that much. For PC3200, it would be a 25% overclock. So you will need to set the memory to run at a fraction of the base speed - this is the memory divider.
An example: Say you have PC3200 memory, which is rated at 200MHz, and you set the base speed to 250MHz to get a 25% o/c on your CPU. In order to keep your memory at its rated speed, you have to run the memory at 4/5ths of the base speed - a memory divider of 4/5. The BIOS on this board is a little limited with memory dividers, as there are only three settings. You can have 1:1, 5/6ths or 2/3rds and that's all. So in this example you can set your memory to 250MHz, 208 1/3MHz (5/6ths) or 166 2/3MHz (2/3rds). The memory dividers are wrongly labelled in this BIOS. 1:1 is labelled as '200MHz', 5/6ths is labelled as '166MHz' and 2/3rds is labelled as '133MHz'. These labels are wrong and misleading.
There are two Hypertransport settings in this BIOS. From context, I think that mp260767 was referring to these two settings and implying that you should set them both the same (I can't think of any reason to set them different to each other, but maybe there is one).
They're in the BIOS as 'CPU - NB link speed' and 'NB - SB link speed'. Again, the settings are wrong and misleading. They are multipliers and not the stated speeds. The settings are actually 1x, 2x, 3x, 4x and 5x, set relative to the base speed. So if you increase the base speed, you must decrease these two settings in order to keep the two hypertransport links within spec (1GHz). To continue the 250MHz base speed example, you would need to set these two links to 4x, to get 4x250=1000. So you'd set them to "800MHz", which will give you 1GHz. Wrong and misleading labels abound in this BIOS.
The common suggestion, which I think is pointless, is to set the multipliers and dividers extremely low (to make sure that the CPU, memory and hypertransport links are running well under their rated speeds) and then see how far you can increase the base speed as far as you can. Since increasing the base speed has no effect at all on performance, I don't see why people do this other than as a competitive sport, to see who can get the highest base speed.