So, I've just installed Silverstripe, which, btw, my mind keeps calling Silverlight, and I installed using the blackcandy theme rather than the blank theme.
If I want to have a second site to follow the tutorial/do my own thing do I need to install silverlightstripe again in a different folder/name, or does silverlight (really, I didn't mean that one) manage different sites from the one installation?
Out of the box, SilverStripe does not have functionality for managing multiple sites from one installation. However, there is the subsites module which allows you to administer many sites from one installation (as the name suggests). However, I dont think it supports much beyond separate subsite database content - that is, all subsites would have the same visual style, and operate out of the same template files.
Therefore, I think the only way to achieve what your looking for is to create a new installation of SilverStripe in a different folder.
As for post previewing - not at the moment, but I think theres some work being done on the forum module as we speak, and this may be one of the new features.