1. DROP THE CR@PPY DiabloTek PSU. Get a cheap OCZ unit which would be MUCH better in terms of quality. The last thing you want is a bad PSU killing your back ups.
2. Everything else looks good. Just get a single 2GB RAM stick.
3. I haven't used Home Server extensively, so I don't know how much CPU power it needs, but you may be better off with a cheap AMD CPU and board compared to an Atom.
4. Again, depending on your skill level, consider running Linux distro (ie FreeNAS,etc).