I recently built a NAS for my family, i used the software NAS4FREE does everything i want it too.
The specs of the system I put together are:
Intel core 2 quad 2.83 ghrtz
4 GB DDR2 RAM
Dunno on the graphics card, bought it for 5$ on trade me. (the equivalent of trade me)
x2 280 HDD's on raid1
System works fine, i can upload and download files easily, and nas4free barley even scratches the system, could probably gone with much lower specs without issue.
However, word of warning, I am not sure about other NAS software, but I do know that nas4free did require a lot of configuration.
P.S. nas4free is basically and earlier community maintained version of freenas, the differences are that it uses less system resources and UPS file setup, rather than ZPS.