This topic is extensively discussed in this thread here:
http://www.cadillacforums.com/forums...-v6-4-a-4.html
Read through that thread carefully, and you should have a complete understanding of the 'issues'!
Short answer: The V6.4 disc will work to display map detail, but on older (Pre-'05) NAV systems you can't enter destinations due to a software glitch. I have an '04 and it does not work in my system. There is supposedly a software upgrade disc that will fix this problem but it's still on backorder with GM.
Hope that helps!