If you are familiar with overclocking what so ever, it is akin to Intel?s FSB, in the respect to overclocking (aka increasing the HTT/FSB frequency to OC a processor on a given multi).
Also Google is your friend for a more in depth explanation (easier to google than explain

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EDIT:
The 1-5x you speak of is the LDT multi, this multiplies the HTT which is set at 200Mhz to give you your 1Ghz Hyper Transport. What you will be looking to do first off is lower the LDT multi down to 3 (because you do not want to exceed 1000Mhz on the HT link). You then have the ability to play between 200Mhz and 333Mhz on the HTT with out worry. I see you have a 165 (as I used to have). If your looking for about 2.6Ghz you will be looking at the following settings, 289HTT x9 (165 multi) = 2.6Ghz, LDT set at 3x which is 289 x3 = 867Mhz on the HT link which is acceptable because it is below 1000Mhz.