I think that for high quality images you will not be able to get 60 minute render times very often, even with a fast machine. Over time the rendering engine will be optimized and this may become more reasonable, but even then a scene of any decent complexity and quality will probably take longer. This is true of most rendering systems. Certainly you can render for example a basic single light non-displaced high poly object in say Maya in 1 minute or so, but add displacement, global illumination, multiple high poly objects, reflections, refractions, etc. and soon you will have high render times there too.
Terragen 2 is simulating a complete and accurate global atmosphere system, volumetric clouds (if used), global illumination, reflections on complex geometry (if water or reflective materials are present), high-detail displacements, and literally billions of polygons of instanced objects (again, if used). Given how much is being rendered it's not terribly surprising that render times are high, especially given the fact that the engine is as yet still not fully optimized and most importantly does not take advantage of multi-core/processor systems. As soon as TG2 is multithreaded you will see an immediate and tremendous speed increase.
In any case the best machine for TG2 is going to pretty much be the best machine you can buy for near any task. TG2 performance is largely driven by CPU speed and power. Currently the procesor landscape looks like this in terms of power, in order from least to greatest: Pentium 4, Athlon 64, Pentium D (dual core), Athlon 64 X2 (dual core), Pentium M/Core Duo, Core 2 Duo. So if you take that and look at the speed of the processor in Ghz and compare it to the "power" of the CPU, you can get an idea of what the best CPU's are. A Pentium 4 at 3Ghz is a lot slower than an Athlon 64 at the same speed because it has less "power". That's a very simplified explanation but it plays out in reality fairly well.
So for a simple and specific recommendation go for a Core 2 Duo system with at least 2GB of RAM. The graphics card won't matter much, just get something decent. I would suggest in particular the E6600 Core 2 Duo at 2.4Ghz. It's faster than an Athlon 64 X2 5200+ and only slightly more expensive. If you have money to burn you can consider the Core 2 Extreme at 2.93Ghz, or the *quad core* Core 2 Extreme at 2.6Ghz (with 4 cores!). But only clock speed will help you with rendering time until TG2 is mulithreaded. Once it is however a quad core would give you amazing performance. But if you're just buying an upgrade to deal with TG2 renders it would be best to wait on a quad core since they will be cheaper and more available next year when TG2 is released.
I hope that helps. If you need any more specific recommendations let me know.
- Oshyan