Don't you have access to the internet?
Searching is easy, and with the variety of search engines, information that you ask about is literally at your fingertips!
http://www.everymac.com/systems/apple/powermac/stats/powermac_7100_80.html
But, to help you out (again!)
MacOS 9.1 is the newest OS
No PCI slots, so no PCI cards.
Memory needs for classic PowerMacs are here:
http://www.chipmunk.nl/DRAM/WhatIsAllowed.htm
(FDO? I'm guessing you mean FPM?)
The hard drive is SCSI (no IDE for the 7100), so you would just need to find a 6GB (or larger) SCSI hard drive.
Here's a 6.4 GB SCSI hard drive for sale:
http://www.welovemacs.com/st64s011.html
If you need to ask, there is no practical limit for SCSI hard drive size, as long as the interface is compatible.
Keep in mind that an older PowerMac didn't need a lot of space on the hard drive. You could boot to the finder from a single floppy disk, probably less than 1 MB on the diskette. A 1GB hard drive was a huge amount of space almost 20 years ago.
In fact, if you choose to use a large hard drive like that, with a system prior to OS 8.1 - you'll discover that a large hard drive (really the partition size) is a real limitation, because there is a maximum of 32,768 files and folders in the partition.
For the difference today - Try to limit your OS X system to less that 32,000 files. I don't think it's possible, when you can easily find Macs with several million! files on one partition.
It's one of the huge (literally) changes with today's technologies, where storage that was unthinkable just a few years ago, is needed, and often much more for backups, and whatever else you have. Moving right along...
