Perhaps I should have re-phrased my questions to you:
Based on past experience (with XP in this case), it matters what is running at slow closedown. Is there a common application or circumstance.? What had changed on your system (like new Anti-virus) when the symptom first appeared? There'll likely be an application or some function that is holding on for a timeout period to as remote link that is causing this.
Did you look in the "Performance Issues" section of the Control Panel Performance Information and Tools function?
Also a browse through the event logs will tell you what's going on during shut down.
After a certain point, increasing the RAM will increase boot/shutdown times.
vista is slow at shutting down because it has to save everything to disk. when its running it keeps what you are working on mainly in RAM, which is why it uses a lot. This makes the data faster to work with but it means that you must shut down properley so it can save it all back to the disk or else you will lose data.
its the same reason why its slow starting up, it caches your commonly used apps to RAM.