The motherboard is the most important part of the system.
The form factor is how big the motherboard is and where the screws are. You must match the form factor with the Computer Case. Some cases support multiple form factors. Regardless, make sure that the motherboard fits the case.
The socket determines what kind of chips will work on the motherboard. You must match the CPU to the socket or the CPU will not work.
The bus speed is how fast the CPU can talk to the rest of the system. Try to match the CPU to the motherboard. If you can't match, then get a weaker CPU and a strong motherboard with bus speed.
Memory comes in different shapes and sizes. Match the type to the motherboard.
- As of August, 2010: DDR3 is the best option. The price between DDR2 and DDR3 is negligible.
Match the memory speed to your motherboard. Buy the fastest memory you can without exceeding the motherboard's speed. Note that the bus speed has nothing to do with the memory speed. The CPU has two separate channels for talking to the memory and the bus.
Max Memory Supported
Be careful not to exceed the memory per slot. Some motherboards can only handle 2GB per slot, while others can handle more.
Many motherboards have on-board video. Here, you want to either ignore what they have, or find a motherboard with a useful integrated video. Don't get excited about the on-board video. Usually, it's a very cheap solution.