At the present time, the encoding speed of DCP-o-matic is almost entirely governed by the speed of your CPU.
We have quite a number of benchmarks, listed here, which might help you to decide what CPU to get. As examples, we see a typical encoding speed of around 16fps with a Xeon 2687W CPU, down to a more modest 6fps with an Intel i7 980X.
As you can see from this graph encoding speed is reasonably correlated with the “Passmark” benchmark, so you can use that to get a rough idea if a particular CPU is not listed in DCP-o-matic's benchmarks.
Aside from CPU, hard disc space is useful as a typical feature film DCP will take up between 60 and 100Gb.
It is possible that in future DCP-o-matic will be accelerated by running the JPEG2000 encoding on a graphics card. If that happens it may be more cost effective to spend money on a fast graphics card than a fast processor. As it stands, though, the priority is the CPU.