What you have not indicated what OS you are using. Any of them will work just fine with a single processor.
With NT of W2k you will have to start over and reload the OS. The project will still be an upgrade. What is not realized is the actualusage of the second processor, it is far less than thought. If you go from a dual 450 to a single 800 you will end up with a much faster machine.
With 95-98 you can boot to save mode remove everything from device manager, shut the computer down,swap the hardware and start the computer and it will detect the new hardware. I have had some reasonable success with doing that.
If you are not using NT/W2K save your money, the second processor is a waste, they are not used in 95/98.
Asfor A MB. I have used ASUS to build machines with great success. They make several Dual boards, depending on the style of processor you want to use.
good luck