I built my own desktop pc a few months ago so i know a little bit about motherboards.
I have found that Gigabyte or Asus are good manufacturers.(i have a gigabyte motherboard)So i'll stick with them while i try to explain what your looking for,without getting to confusing.
You have an AMD Athlon CPU so your looking for a motherboard that has a socket AM2 or AM2+ for your processor.novatech.co.uk or microdirect.co.uk are good retailers online for finding the motherboard your looking for and are resnably priced. You didn't give the specifications of your RAM though? As all new motherboards either have DDR2 or DDR3 RAM,so you'll most probably have to upgrade this as well!
Also alot of the motherboards do not come with onboard graphics cards,so you could be looking for one of these as well! and if you want HDMI you want a graphics card that has an HDMI socket as well as a monitor socket. I have 2 RADEON HD4670 cards installed on mine,the HD means high definition. Hope this helps cap42