What to look for
Few PC based Spice programs use more than 2 or 3 cores because it is hard to efficiently partition the algorithm Spice uses to solve the non-linear circuit matrix. For example, 5Spice uses two cores.
don't buy the low cost processor option. Intel and AMD have low cost processors that have respectable clock speeds but less efficient internal architectures. They are designed to be cheap and to run real world apps significantly slower (so you will buy the more expensive ones).