BUILDING YOUR OWN PC?

Hi,i am considering upgrading my old pc which has an Abit NF-95 motherboard socket 939 and AMD Athlon 3200+cpu,i wondering would it be better to buy an ATX motherboard instead of mATX and how easy is it to build your own pc including drives power supply and heatsink fans etc.i have a fair idea but i suppose lacking in a little confidence.Could someone advise me.Is it simple to build your own.Thanks.