That is absolutely crystal clear, but I've always assumed (maybe wrongly) that as soon as the HDD can be seen by the TXT-mode part of XP setup, it will also be seen by the GUI-mode part.
MassStorage TXT-mode drivers are needed in early mode of TXT-mode of XP Setup to allow you to detect your MassStorage device,
so that you can select where to install and let XP setup copy the files (otherwise harddisk would be invisible).
So this mechanism only loads more boot drivers that allow XP Setup to see your harddisk.
This also seem to imply that TXT-mode and GUI-mode parts of XP setup need different drivers. But imagine a plain vanilla XP setup (no driver packs included), to be installed on a SATA HDD. Isn't it enough to provide drivers with the F6 floppy ? I mean "enough" in the sense that both TXT-mode and GUI-mode parts of XP setup will use that very same driver.
For the timing of Install I can say that offering the Chipset and MassStorage drivers too late would mean
that XP Setup has in GUI-mode no access to your harddisk.
Therefore it sounds to me as if integrating MassStorage TXT-mode drivers to XP setup is definately a must-have, but all the rest can be installed as late as desired (included post-install); however I understand that too early is not a good thing.