This guide was written for the P5B series of boards which used the 965 chipset, your board is the P5K series which uses the P35 chipset.
Click the P5K addon guide in my sig.
This first part of the guide is worth a read before you read the P5K add on, just for general info, all the question you have just asked, are answered in the guides
The memory frequency (when set to 1:1) will always show double the FSB, so and FSB of 266 will show a DRAM frequency of 533, and 300FSB will show DRAM frequency of 600 and so on.
To enter the FSB frequency, just highlight it and then type in a number, then press enter - simple as that.