Not sure how to access the old threads.
Yes, there are 4 different lines. You can compare them all at
www.mbquart.com . They also have a dealer locator on the website.
As far as which ones to get, it all depends on how much you want to spend! Your local MBQ dealer should be able to help you out. You may want to decide which ones you want then search online. Probably find a better deal, but may have to install yourself.
Check this link
www.nextmill.net/bmw540/ This guy did a speaker upgrade and has pics and all showing the work. I found that link on
www.bmwtips.com. Another link that has speaker upgrade articles is
www.unofficialbmw.com/e39.html . This page seems a bit out of date, but you may find some good stuff there too.
Good Luck.