If you want to use the OpenDNS servers, then the internal DNS is not going to matter since the DHCP takes care of anything, so you it might be best to leave the computer settings as they are :P
On the router, the normal setting for DNS settings is set to automatic, meaning it picks up that data from the ISP and the ISP assign the DNS server. All you need to do is manually override it and add the OpenDNS server addresses in and you are on your way!
Thats all the configuration you need to do. I don't have any experience with Draytek, so I hope you find your way round!