Terragen does not use the graphics card for primary rendering, but faster cards *do* help with speed in the 3D preview, which is getting increasingly complex. The newer textured mode for example. So that's worth considering. Getting a high-end card is certainly unnecessary, but something better than onboard is probably ideal (especially as onboard solutions tend to have more driver issues, sadly).
Most of TG's rendering process is now multithreaded, including populations (as of TG3.0), GI, and of course primary rendering. So more cores is generally better. Intel CPUs tend to perform better for Terragen, especially when considered "clock for clock". But the price/performance ratio is a more open question, i.e. for a given price which system will give you better performance. That being said Intel also tends to run cooler and be more overclockable, if that's something you ever want to consider. For rendering running cool is of course important.
I agree on 16GB of memory as a minimum. Get it in 2 8GB memory sticks for easier later upgradeability.
- Oshyan