You may get away with a Reapair install, but it's always better to start fresh - the new MoBo will have a completely different chipset etc, so it's always best to go that way.
Note, you won't be able to use the RAM from the old PC no matter which way you go - it'll be PC100 or PC133, and modern boards use DDR or DDR2.
You'll probably need a new Power Supply as well, as I doubt the old one will have the right connectors, and probably not enough GRUNT.