You will almost certainly need a new motherboard and CPU, probably a new power supply as well. There may not be many components of your existing machine worth keeping, to be honest.
If you have the money I think the "sweet spot" for price/performance right now is a 6 core Intel CPU (yes, Intel; they've got the fastest CPUs right now, benchmarks don't lie

). The best thing to do, I'd say, is look at the Terragen 3 benchmark results and then check the price of each of the top performing single CPU systems that are not overclocked (overclocked systems are marked in red):
https://docs.google.com/spreadsheets/d/1eX9Ltn3_9BjsamA0Pxeflv5AKrjkgViEY8VuetB8e3k/editAnd you don't want a Xeon. So for example the Intel i7-5930K, 6 cores, 3.5Ghz, $580 and a benchmark time of 5.5-6 minutes. There *is* a single AMD FX result up there with the 5930K but *none* of the other AMD results of a similar CPU are anywhere near that, so I have to assume that result is not correct (which is why it's highlighted in yellow; we'll follow up with the person who submitted it).
For RAM definitely get at least 16GB, and RAM is currently pretty cheap so I'd say go for 32GB if you can afford it.
Graphics card will actually be even less important in TG4 than in TG3 since the new ray-traced preview is all based on the CPU, whereas the current 3D preview is OpenGL, which uses your graphics card for many functions (mainly object drawing). It may still be useful to have a reasonably good video card, but it's far from the most critical component. Any mid-level current card from Nvidia or AMD should be fine, don't spend more than $150 on it.
Also if possible I would recommend an SSD. They're pretty affordable these days and it will boost overall system performance. It won't help TG performance much, admittedly, but you'll appreciate the super fast boot-up speed, app launch speed, and working with any files that are on the SSD. The overall operating system performance is just better.
- Oshyan