When buying graphic card for pc, what is PCI-Express? what means "require AGP"?

PCI Express is the newer format for Cards, it is the slot it sits in. AGP is the previous slot type for Video Cards. You need to know your motherboard type and find their website to find out what type of slot you have for your card purchase. Also AGP have speeds 1x 2x 4x 8x so make sure if you get one of those you get the correct type of that also.