According to RFC2182 you need to have at least 3 nameservers that are not in the same subnet and preferably geographically diverse.
Like Chopper3 I wouldn't recommend consumer gear for nameservers. But then I really wouldn't recommend running your own servers anyway. I would recommend that with these numbers of queries that you rely on a managed DNS service as teams like mine have worked tirelessly to design these services to take incredibly high numbers of requests.
If you really want to run your own dns servers, rather than rely on a managed DNS service, then at a minimum I would suggest you look into running your nameservers either on some VPSs (Virtual Private Servers) or if you like hardware get a quarter rack in another Colocation DC in another city and sticking one pizza box (1U Rackmount server) in there and two in your primary DC.