Terragen relies primarily on CPU and memory. The GPU is not currently used at all for rendering and is only important when using the "old"/standard OpenGL 3D Preview (as opposed to the newer RTP, Ray-Traced (or Real-Time) Preview, which is CPU-only). In the future we may take advantage of the GPU for the RTP, but it is not likely to be used for final rendering any time soon, so CPU is most important.
CPU has the greatest effect on rendering speed by far. Multi-core CPUs are taken advantage of and Terragen scales well up to about 32 cores/threads, after which it can be better to render without Hyperthreading enabled, or to run 2 or more parallel instances to take best advantage of available hardware. We are constantly working to optimize and improve multithreading performance as well.
Memory is important for dealing with complex and detailed scenes, and higher quality renders. Having more memory is increasingly important with Terragen 4 as the newer rendering algorithms, as well as the RTP, both require more memory to work well. Terragen 4 can render quite a bit faster than Terragen 3 when objects are involved in the scene, but to do so it requires more memory. If you are buying a new machine I would recommend 16GB at a minimum. Memory is relatively cheap so get 32GB if you can. You may not need more than that for now, but leave room to upgrade, or splurge on 64GB if you're getting a multi-CPU system with lots of cores/threads because you also need more memory per-thread, and more powerful systems are likely to be rendering more complex scenes and thus require more memory as well.
If you're looking to buy something off-the-shelf that's fast and reasonably priced, consider any 6-core Intel CPU, or potentially an 8 core AMD (not as fast but cheaper). If you are willing to DIY a little, then here's some info on an *amazing* deal for a really high performance workstation build:
http://www.techspot.com/review/1155-affordable-dual-xeon-pc/The deal on used Xeon CPUs is really pretty incredible right now...
- Oshyan