Render nodes install just like the regular workstation/"interactive" license you probably already have setup. Just use the same installer, but on your other machine(s). You can only use 1 machine at a time for interactive (scene design) purposes, but you may use any of them up to a total of 4 (including your interactive license) for rendering.
The systems would need to be networked unless you had some other way of transferring scene files for rendering to them (e.g. flash drives). There is no built-in network rendering feature in Terragen at this time, so you either need to setup renders manually (not too difficult if you only have a couple of machines to manage, for animations just split the frame range between the machines, e.g. frame 1-10 on machine 1, frame 11-20 on machine 2), or you can use a render manager. A list of compatible render managers is in our wiki documentation: http://www.planetside.co.uk/wiki/index.php?title=Terragen_Resource_List#Render_Managers
Terragen does not use any GPU functionality for rendering at this time. It is 100% dependent on CPU power (RAM availability is important, but does not significantly affect render time unless you run out and it is using disk-based swap). The faster your CPU and the more cores you have, the faster your render. Older systems still work fine, but you'll want to try to account for the difference in speed when splitting up renders. The best way is to run a benchmark scene (render the same scene on both systems) and work out the difference in times. Hopefully it's something simple like your slower machine is half as fast (twice the render time), so then you just assign it half the number of frames as your main machine and they should finish in a similar amount of time.