Toadster - I don't think that board you linked or the case is an appropriate choice for a NAS machine. The case only allows a single hard drive, and the board has no native SATA support and only a single PCI slot which means you're stuck with 10/100 ethernet which, to me, is unthinkable for a NAS to which you intend to copy any significant amount of data.
I will humbly refer back to my own post above in which I identified part by part all of the components you need to build a $500 do it yourself NAS/server that includes 1 TB of RAID5 storage, Gigabit ethernet, a gig of RAM, Athlon 64/3500+ CPU plus a decent case with PSU with room for all of the drives. I'm running exactly this configuration at home (added an optical drive and a 40 gig IDE drive for the O/S) right now using Windows XP Pro, though Ubuntu would probably do the job also. There are not a lot of options since you are looking at a $500 budget, $300-$350 of which goes into drives leaving you only $150-$200 to afford a case, PSU, processor, RAM and mainboard.
For $500 there are no "out of the box" solutions that will fit your budget and your DIY options are exceedingly limited. Unless you can find another sub $50 mainboard with Gig-E , native RAID5 support, and integrated video the Foxconn offeringI linked is your only option. The AM2/775 mainboards that support RAID5 are simply too expensive and going hardware RAID is not an option either at your desired pricepoint.