Actually FAT32 has a 2TB limit (According to fdisk), but ATA-100 has a 137GB limit, and ATA-133/166 250GB
I'm not sure about the limit of HFS+, but heard that you should not have drives over 20GB, and if you use Large files (>1GB) like those from VPC, you should use HFS, because of Faster Access (or so I'm told.) I'm not sure about the specs for NTFS or UFS, but I believe their big advantage is encrypting ability, more organized structure, and even faster response times.
To add a harddrive to you system an make it seam like there is just more space, you should be able to setup a RAID with Striping. I'm not sure if it was here, but a guy set up a 5MB drive bu RAIDing 5 Floppies together. I'm not sure about doing it after the fact, so do some research. BTW: OSX has build in RAID functioning.
TEG