While I have no intention of reproducing the excellent installation guide that's already online at http://www.openzaurus.org/oz_website/content/installguide or the entire FAQ section at http://www.openzaurus.org/oz_website/faq/browse_cat?id=5, I will discuss your most important installation decision, which is choosing a zImage file.
For both the 5000D and 5500, you have to choose a zImage file that represents how you want to partition your RAM into operating system memory versus additional filesystem storage. Remember from the previous section that there are only a few megabytes of free space left in the 16M flash after installation, so it's likely, if you have no CF or SD card to supply additional memory, you'll have to allocate at least some of RAM to file storage.
For example, the 5500-based "40+24" image file will allocate 40M for O.S. memory, and will use the remaining 24M to create an ext2 filesystem. Regardless of what size you select for this filesystem, it will be available under the mount point /mnt/ram, and can be used for additional file storage, software installation and so on.
Note that whether or not you plan on adding extra storage with, say, an SD card will affect how much of RAM you decide to allocate for file storage. In the extreme case, if you have lots of removable media storage, you might opt for either the "32+0" image or "64+0" image, giving all of RAM for O.S. memory and using only the storage in flash and on your storage cards.
(While it's certainly possible to extend your Z's storage with a CF memory card, given that the CF slot is also used for devices like network cards, digital cameras, GPS units and the like, I'm going to assume that you'll use only SD cards for additional storage).
If you choose to install either a 32+0 or 64+0 image and work mostly with your SD card, you'll almost certainly want to move some of the major system directories from flash onto the SD card first. All of this will be covered later in the section on filesystem and memory management.
Don't forget that, regardless of which image you install, you'll still have a few megabytes of available storage in writable flash to play with. Granted, that's not a lot, but it's still available.