I am not going to answer your question, directly, but will say that a clean install is always better. Go with the clean install. You do not save time with an upgrade if you take into account the multitude of problems that may occur.
Back up your stuff, use Windows Easy Transfer and all you need to do is reinstall your software and you have the very best installatio, a clean install.
Enter your key after you have used the system for a few days, your software is all installed and you are satisfied, you have 30 days, which can be extended. Clean Install From Upgrade Vista
Would you want your old car after being detailed or a brand new 2012 car, it works the same with the OS.
If you choose not to use my advice, you need to have SP1 with an SP1 install.