SEO advice comes in two types.
Unfortunately it is difficult to tell the difference a lot of the time (even when giving it). Google webmaster tools contains a lot of tools and information for budding webmasters so is probably worth looking at.
In regards to your question. I believe that a website can accrue "google juce" which will support any pages on it. So if you have a site for each product they will have to stand alone, where as if you have a good site with many products on in they will support each other a little in terms of page rank. (This is a very hazy area but from my experience seems to be true.)
For example once there are lots of inbound links and content on SuperUser new pages will need less inbound links to get a high page rank because they are on this site. An identical page on another site with the same number of inbound links would not score so highly.
So I would suggest that one site would be better from a SEO point of view.
But to start with I would worry most about the basics, having SEO friendly HTML, a navigable site and lots of good unique content.