The first x86 project I saw doing line speed route+filter on 10gpbs used 4x top-end Nehalem chips, an output of the RouteBrick project

Although, their original paper says they used a 2-socket prototype and got some very impressive numbers: https://www.sigops.org/s/conferences/sosp/2009/papers/dobres...

So maybe you could skate by with a slightly cheaper machine ;)