Normally you'd just put the router's LAN address in there and rely on it picking up your ISP's DNS servers when you log in.
I tried the OpenDNS servers against my ISP's own (putting them in the router, not the computer)... the primary was slightly quicker than OpenDNS, the secondary was slightly slower.
And I mean
slightly, sub-millisecond, which could easily have been due to traffic variations.
So I just let the router pick them up from my ISP.