If you are following the standard security practices, then your default firewall policy will be to block everything. All you should have to do is write a rule to permit tcp and udp traffic to port 53 if you want to permit incoming DNS requests.
The traffic you are talking about is UDP. UDP is stateless. This means that people interested in saturating your connection can send the packets to your address even if you just drop them. Still you may be able to do something semi-useful with the iptables recent match, to only allow a limited ammount of traffic to actually be accepted and processed by the system. Evan has a example of the usage of this for SSH here. We might have to see your entire firewall rule set to tell you what rules would have to be added.
If you have a serious DoS against your system, you would almost certainly need your ISP to help you, trying to deal with a flood with a host-based firewall on a VPS will really not be very useful.
If you don't have it already, you should consider setting up a few secondary DNS servers for your zones on a completely different network.