500gb, 1tb, 1.5tb, 2tb, 3tb?...

2 TBs.

Ditto 2Tb

1.5TB for the main backup drive (installed in PC)  and 1TB for secondary USB connected.   

Which one?
2TB for sample drive backups, 1 TB for OS drive backups, 1.5TB for data drive backups.
I use rotating backups for each drive so at all times, I have 3 chronological backups on each drive that I can restore from.

I just picked up a 4TB drive. I also keep 3 backup copies of stuff for when drives die, which they will do. I don't trust anything fewer than 2 backup copies. My general method is to buy a huge backup drive when the price seems right, and then consolidate all the previous backups onto it. Then I take the oldest backup drives that I have, (which at this point are the 500GB or thereabouts ones) wipe them, and get rid of them.

At this point, I have about 2.5 TB of backup data that I'm hauling around. So a copy is on my new 4 TB drive, two 2 TB drives, and three 1 TB drives.

local 8 x 1TB raid 6 with a duplicate of same..
NAS: 8 x 2TB Raid 5, the raid 6 backs up to the raid 5 NAS nightly
once a week a 2TB eSata leaves the building..

once you lose data you tend to get anal after..
sorry had to brag!

650gb for some bits and 1TB for other bits.

1Tb and 2Tb here

2 TB to me.