The motherboards you need to watch out for are the MATX boards, they fit in full size cases and work, but they come with 3 or less PCI card slots. it's what I have, and it works, but when I build my next 1, I'll be using a Full ATX board.
mine's 5 years old now, and I have upgraded once a year.
AMD sempron 2500+ ( 1.75 ghz )
512mb ram
128 MB AGP graphics card ( Nvidia 440 MX )
30gb Maxtor
160gb Western Digital
Lite On DVD RW DL w/ lightscribe 20x speed
I think I might have around $600 in it now after upgrades
I built almost the same computer for sombody else a few months ago, all he had was the case and cd rw remaining from another computer. I think we had $130 in parts
I use
www.tigerdirect.com for parts myself but
www.newegg.com is good to