In a nutshell, Superfetch caches your most frequently used programs into RAM in order to, in theory, open them faster. After all, RAM is faster than mechanical hard drives, right?
The drawback of Superfetch is that startup tends to be slower--Vista has to take time to reload your Superfetch cache back into RAM.
Typically, I disable Superfetch on any Vista machine running less than 4 GB of RAM (in other words, 32-bit Vista, although I've seen some 64-bit builds run less than 4 GB of RAM).
Disabling Superfetch means that startup should be a bit faster, but opening huge programs may be a little slower (might not be noticeable until you try disabling it on a machine with 24 GB of RAM running AutoCAD or Revit).
In general, if your 6 GB of RAM is being chewed up, it's probably Superfetch.
When you stopped the Superfetch service, did you remember to disable it? Did you reboot afterward?
...and finally, when you open Task Manager, did you click the button that says "Show processes for all users"?
