File server solution for a SME

A 50 user company is looking for a simple 4TB (RAID 5) network storage solution of type NAS. It needs to be place on a rack.

The users will use this storage everyday all the time and will proceed to a backup every night.

What type of resources ( CPU/memory ) a file server require to support this amount of activity ?

The company doesn’t want to use any Windows services of type active directory yet.

I was directly thinking of the QNAP products because of its simplicity and ease of use.
The TS-439U-SP ( http://www.qnap.com/pro_detail_feature.asp?p_id=119 ) seems to have everything they need. The problem is that its resources ( ATOM 1.6Ghz + 1 Gb memory ) doesn’t seem that high for 50 active users.

What do you think about this solution ?