-Which OS can best be installed first, CentOS or XP?
XP u need to install first, boot with Xp, create the need partion, format NTFS, install, cause win XP boot loader over writes yr GRUB boot loader, save the trouble, finsh MS first
-Are there special actions required to enable proper dual booting?
Make sure u install GRUB on MBR, so it will overwrite XP bootloader and will give u choice which OS to boot.
-Which boot manager can be used best CentOS' one or XP's one?
CentOS - GRUb
-Can CentOS read (and perhaps also write?) from the NTFS partitions?
Yes but u need NTFS support, get the RPM from http://linux-ntfs.sourceforge.net/rpm/index.html and u have rw access to NTFS
-Hypothetically: would installing VMWare and installing XP in that give me pretty much 100% XP compatibility for at the very least the various web browsers such as IE, and also for XP Office?
Yes u can move in with VMWARE also, same stuff like a OS installed in sep partions, but vmware costs $$, if u dont mind shelling out, vmware is cool, otherwise dual-boot rocks.