if you are looking for net and video its going to be hard outside of what is already standard, I picked up a dual core 1.6ghz atom micro atx with everything but ram on board for 60 bucks shipped on ebay, it runs windows 7 x64, and has meh performance with 720 video full screen
add ram and maybe a pico power supply and you can run a balls out netbook for ~100 US$ off of a car battery in a large cigar box for a month
otherwise arm based systems