ameen is right, and transfer is all based on requests anyways. One domain that recieves no hits will not have very much DNS traffic. One site that recieves a million page views a day will have a litlte bit of DNS traffic In any event you will not ever have to worry about the 3mb/s line unless somebody wishes to DDoS your DNS Server.
You could even get by with a simple AMD 400Mhz system with 256mb and still have room to fly if it is just working as a DNS Server.
Your traffic level should probably not be more than a couple of bytes each time the DNS Server is used and it is not going to be so intensive anyways.
You don't really need RAID or an unmetered bandwidth. You probably don't have much to RAID about unless you are doing DNS Hosting as a business with tens of thousands of sites. If the details do not change regularly, a simple daily backup should suffice.
Even if you have something like 5gb/mth traffic, it would still be an overkill in most instances.