The target machine has XP (SP2) installed, and the goal is to install Ubuntu 9.04 as a dual-boot. Ten years ago, this would require preparing the disk with something like Partition Magic, but I think there has been progress. Am I likely to need anything besides the Ubuntu 9.04 Desktop CD?
Go get the latest ISO from Ubuntu.com (unless you have a need for more than 4Gb mem, get the 32-bit version), unpack it onto a memory stick then install away (well reboot your PC!) - just make sure there's enough space for your Ubuntu installation.
One of the early parts of the setup process is a "Partition Magic"-dual-boot-builder-on-steroids interface.... Makes a mockery of how we used to do this in the 'old days'!!
The Ubuntu 9.04 Desktop CD is enough.
section: Resizing Partitions Using the Ubuntu Installer
Just to put my two cents in... If you have the ability to use 64-bit version, go ahead and do it. Not having the added memory will of course not get you all of the extended functionality/performance, but those programs that are made to take advantage of a 64-bit processor will run faster than in a 32-bit OS.