My experience, based on (so far) fairly modest sample sets:
My C: drive (SSD) has the OS, Windows 7 Ultimate, HW program and a few small ancilliary programs and takes up 48 Gb.
My D:drive (conventional HD) Has the installation files for one fairly big (Hereford Willis) and a selection of smaller organs and uses about 100 Gb.
My F: drive (SSD) is only used for organ caches an is currently using 38 Gb.
The D: and F: requirements will vary enormously according to the the user's taste (and pocket) for large organs. Personally, I'd think an additional 4 TB HD would hardly be necessary, but other users will probably have different views.
Nick