Beside knowing how to replace a motherboard.What do I have to know to replace one beside the size of it? I have 2 older computers that have bad motherboards.They were made around 2002 and are Dells. Dell wanted around $300. For replacements.Is there a website to buy older generic motherboards at a fair price?

For any period, the motherboard, processor, and memory can be considered a set of given vintage. That is, when replacing the motherboard, often the processor and memory need to be changed as well since the processor socket format and the memory form factor have changed over time. When replacing older motherboards, it is hard to find currently manufacturered boards that will work with processors or memory 3 years old and practically nonexistent for 7 years old. Therefore you have two choices, find a used surplus board compatible with the old processor and memory, or upgrade all three to current technology. Upgrading all three is often comparable to the cost of just the board from vendors who stock vintage replacements, as you discovered. My personal preference is to upgrade all three to current technology as buying obsolete tech doesn't make sense to me (it also avoids compatibility issues with vendor specific memory), however given the age of your machine, you will likely find it has parallel ATA hard drives and CD drives. Many current boards have serial ATA connectors for the hard drives and a single parallel ATA connector for optical drives. The two ATA formats are not physically compatible, and it is another factor to be considered while looking for a board. Adapters are available but that just adds to overall cost. Another potential gotcha is that the power supply may need to be upgraded for power capacity and connector format to match current hardware. Also some Dell supplies are proprietary.

My cut off point for trying to match old hardware is 3 years - longer than that and it is very time consuming to find suitable replacement parts. I usually begin a board search by selecting compatible boards with the features needed from the information on motherboard manufacturer's web sites. Once a few board models are selected, I then Google the board model to find a seller. I haven't found a one-stop "Boards'R'Us" type site.