When I ran a check of my components through AIDA 32, I find that the default FSB for my Athlon XP 2600 is 100mhz and the RAM is DDR PC3200 with rated speed of 200mhz. However, I can change the CPU speed in the CMOS to 166 without any ill effects(this is apparently the highest since a change to 200 prevents booting altogether).

So is maintaining the speed at 166 fine even though it's above the default 100 mhz speed and more than half the DDR speed?

The default FSB value for the BIOS is 100MHz, not your CPU. All AthlonXP's run the FSB at 133MHz or greater. You could very well have one of the 2600+ versions that run at 166MHz FSB. Do you know what core your CPU has?

You can run your RAM in sync with the FSB or run it at stock PC3200 speeds, it's up to you.

Though my knowledge base is much more humble that saphalline's, my comment would be as follows. You may probably be aware that you could have one of three flavours of the XP2600+; a 133fsb, a 166fsb Thoroughbred, or a 166fsb Barton. If you bought your cpu recently the likely hood is that it is a 166 T'bred and the matching mem would be pc2700. Unless you want to overclock your system by way of the fsb or feel that in the future higher speed mem would be required then pc 2700 would be fine. As regards o/c your fsb your mobo may not be able to handle 200 if it does not have the appropriate divider or some form of pci lock as is available on the nforce boards in which case o/c will push the agp and pci buses out of spec. As saphalline says the bios generally defaults to 100 just in case.

Hope this helps