My mobo is an ASUS A7N8X2.0 Rev 2. The advertised FSB is 333 MHz. My CPU is an AMD Athlon XP 2600+. Presently my BIOS shows the following:

CPU Ext. Freq.                      133 MHz
CPU Multiplier Setting           AUTO
CPU Freq. Multiplier              12.5x
System Performance            Optimal
Memory Frequency               AUTO

Is this the best setting I can get? Can I jack up the CPU Frequency to 166MHz without drastically affecting stability and temperature?

Bubblehead.

That is strange.  What does it say your FSB is on boot up? if tis saying 266 your running slower than you should be.

The XP2600 FSB is 333 (166Mhz) Try setting it to 166, BUT first find out what ram you have in there.  If you have ram that is 266FSB (133) do not do it as the ram will not overclock this much.  If you do have 333FSB ram than I would have no reservations about running at 166MHz as this is what your ram and CPU should be at.

It appears the CPU Mulitiplier setting is on Auto so it probably isnt taking into account the 12.5x

Ther will be somthing somewhere CPU/Ram interface or ratio...... to get your PC running best this should always be at 1:1ratio

There are programs to find out what your ram is but I cant remeber them off the top of my head, but like said check your Rams FSB if its 333 up the CPU Ext Freq, a lot of mobos come set low just incase you put slower ram in.