Socket 478 is an older chip now. If you want a pentium 4 get the lga socket 775. If you go amd get the socket 939. All the newer boards have pci express 16x. You will still see some older motherboards on the net with agp. Do not get a motherboard with agp.. Make shure it has pci express like I said.
Good motherboard manufacturers are Abit, Dfi, msi, Gigabyte.
Try and stay away from celeron and sempron chips they are the cheap chips from intel and amd. They have lower performance and lower cache on the cpu. The semprons and celerons are so-so but I wouldnt get one for myself. What are you going to be doing with this pc?? Any games at all?? So you need to decide on a video card also. Give more info and the video card is easy to pick out.
Are you on a budget also, if so around how much??