Generally speaking, people are delighted when given the chance to upgrade to a larger hard drive, especially because the larger model is usually the latest and the fastest as well. But you might also remember the problems we've faced at certain capacities in recent years - for example, the 528 MB barrier (at 1,024 cylinders) could only be broken with the introduction of Logical Block Addressing (LBA). Later, FAT32 was introduced as an alternative to NTFS (Windows NT/2000/XP), in order to allow partitions that exceeded 2 GB.
At 4 GB, most BIOS versions had problems addressing drives. The same happened recently at 32 GB, and something similar is waiting to happen at 128 GB (limit of the ATA specs). What these addressing problems have in common is that they can only be solved by updating the BIOS (which, unfortunately, doesn't always work), or by replacing the IDE controller. This could be an argument for buying a 100 or 120 GB right now, since bigger models (> 128 GB) will likely cause some problems with computers that are already several months old.
There's something else that can easily annoy the more impatient user: formatting a 120 GB hard drive. The Windows quick format functions are fast, but they do not test the functionality of each sector - Windows 98 in DOS mode does this for standard FAT32 formats, and Win NT/2000 does this during OS installation for NTFS. The latter method is the only way to make sure that the drive works properly, but the downside is that it takes more than half an hour. This is something to consider when planning to install such a hard drive.