The configuration will really depend on your budget and intended use. If it’s really long simulations or 24/7 usage, then you’ll have to go with some server grade equipment. If you only need simulations from time to time, then look for a high end consumer grade computer (like a gaming pc for example).
The most crucial component is memory. You’ll need plenty (at least 16 GB, even more if you need to simulate bigger domains and finer meshes). To run really lengthy simulations or sweeps, you should look for so-called ECC memory. It’s more expensive, but the memory can correct small errors that are likely to happen from time to time (especially when running 24/7).
Since we can use multicore processors, the more the better. You should look for processors having the most cache memory as it will increase the computation speed by a non-negligible factor. That’s why we recommend Xeon-type processors more than mainstream i5’s or i7’s.
If you have the budget, then a great way to speed up the process is by using a cluster of networked Linux x64 computers.