More aboutintel atom n550 celeron e3400 file server
Either one will work fine, I'd go with the celeron for the cheaper price, more board selection (which means you'll be able to find boards with 6 sata ports), and be able to address more than 2gb of ram (the limit of the n550) should the need arise.
Celeron, 4GB of ram.
Recommend M$ SVR 08 R2 SP1 x64. or wait for new version of M$ home server.