Where do you find out what processors are compatible with your motherboard?

The best way to find suitable processors for motherboards is to check the specs on your motherboard. Note that each mothermoard has an appropriate CPU socket. For example, the computer site http://www.allpcdeals.com/servlet/the-Motherboard-%26-CPUs/Categories lists their motherboards by socket, so you can look up the motherboard you have, and note it's socket, and then purchase the processor that fits. The most popular is the CPU socket 478 and LGA 775 (the ladder now becoming the more popular).