You might not need to reinstall the OS, but if you start having trouble not having reinstalled is one possible cause (eliminate it by reinstalling - format...don't install on top of old OS).

If your new mobo has a different chipset than the old one you should reinstall, although some people will tell you to just install the needed chipset drivers (why have sleepless nights? a fresh installed OS puts you and your system at ease ).

First of all read the mobo manual. It's amazing how many people in this day and age still acquire mobos which require you to set the FSB via jumper...stone ages i tell ya :P
Alot of the cheaper manufacturers are totally incapable of generating a half-way decent manual and connecting LED/reset and power wires can be a PITA.
Once you got everything setup (including FSB jumper - if applicable)
don't forget to enter the BIOS on your first boot (some people forget and find they have trouble rebooting (CMOS reset might be neccessary).
You will need to setup your HDs properly (autodetect oughta take care of the drives), boot sequence (i recommend floppy - CD - HD), CPU setup (make sure it has the right FSB selected - alot of palomino users boot up at 100 FSB by default), Video related settings (primary display AGP, AGP apperture size - should be half your system RAM or be the same as your video card, it's a matter of opinion really), desired power options (i usually turn off all power saving features - it will be the day when they actually get standby mode working properly ).

Personally I'd format/reinstall, the best way to get rid of old drivers, unused shared libraries, those hundreds of megabytes of temp files hidden away someplace under My Documents (not even norton cleansweep catches that junk), the virus you caught browsing all those adult websites ( )...it will make sure your system performs at its best (remember to tweak some annoying things in windows, such as error reporting and system restore - that's what we got ghost for isnt it? ).