Apple defines 80GB as 80,000,000,000 bytes (for the iPod and other hard drives), which translates to 74.5GB when you format it and attach it to a computer.
Why? Hard drive manufacturers define 1GB as 1,000,000,000 bytes (10^9) while software vendors define 1GB as 1,073,741,824 bytes (2^30).
You can expect to have 75GB available on your 80GB iPod, but not due to "formatting" or "system files." It's due to the left side of computing (software vendors) not agreeing with the right side (hard drive vendors).
Who's right? Who's wrong? Who cares? I don't. I just wish everyone would agree on a single definition of gigabyte, megabyte, or any other byte.
And no, I will not call it a "gibibyte." That's just stupid.