Adding more RAM is unlikely to affect performance (rendering speed) unless you have been using up all your available memory (with 8GB it's possible but not especially likely). It will however allow you to make much more complex scenes and render at higher resolutions.
Render times will be rather long for higher resolutions, and your detail settings are also fairly high, particularly main detail at 9. I think you would do just fine with 0.75 or even 0.6, especially with the type of scene (fairly dark) that that tutorial produces. Your plant quality, which probably dominates scene detail, will be based on your antialiasing settings (unless for some reason you disabled raytracing of objects), so the main detail is only affecting terrain quality (which you can't see very well, presumably) and GI. 0.9 is probably a waste in this case, if not many others (I seldom go above 0.75 myself).
Supersample Prepass will add some to your render time, but not (in my experience) tremendously. It is generally a good option, though not strictly necessary. I tend to leave it on and adjust other settings instead to conserve render time as needed.
DPI actually has nothing to do with image resolution or detail. It only has relevance when printing, generally speaking. Changing the DPI of your image should not change how it is rendered or displayed. This is why TG does not give any control over DPI settings. DPI is merely a number encoded in meta data for an image. If it's not filled in, many imaging programs will just assume 72dpi as a default. So TG images quite possibly show up that way. In any case the absolute rendered resolution (e.g. 1600x1200) is all you need to worry about when setting TG render resolution. If you want to know how big it will print, divide it by your intended print DPI (generally 300) to get a number in inches.
- Oshyan
