To answer your question yes.
OEM = no upgrades, only new installation, whether you boot up from the cd or insert the cd in a previous OS (like 2000). For example, if you insert the OEM cd the option to upgrade will not be available and you will have to choose new installation. You will still have all your files (docs, music, etc) but you will have to reinstall your programs.
IMHO though, i'd always go for the oem version (cheaper). Just make sure your files are backup up (2nd partition, cd, another computer, etc), and boot off the cd to do a clean install. Clean installs always run much smoother and usually with no problem....
But an upgrade version would be ok if you want to spend a little more money to keep your applications functional and if you don't want to worry about backing up your files if they're on the same drive as the OS.