I have two iTunes servers. I used to have everything on Server #1, but my wife and I got tired of weeding through back episodes of SpongeBob and Power Rangers, so I moved my kid's stuff over to a old MacMini.
These both stream to three ATV2s.
Server #1 - PM G5 2x2Ghz. 4x1TB RAID array.
Music - 31858 tracks, 85.7 days, 690.67 GB (Almost all ripped direct from CD to Apple Lossless)
Movies - 277 items, 8 days, 138.03 GB
TV Shows - 2567 Episodes, 43.7 days, 836.73 GB (All ripped direct from DVD)
Podcasts - 1178 items, 17.8 days, 23.50 GB
Server #2 - MacMini 1.13Ghz, External 1TB HD
Music - 0
Movies - 4 items, 6 house, 4.92 GB
TV Shows - 1598 TV Episodes, 20.7 days, 437.78 GB (All ripped direct from DVD)
I add this up to 2.13 Tb if you got with 1000 math or right around 2 TB even with 1024 math. (Google it if you want to know more.)