I'm glad you are still getting new stuff! I think you have been with us longer than most subscribers so the fact that you are still learning says a lot.
On to OPC. OPC originally stood for OLE for Process Control. I understand that now it has expanded its role so much from its original purpose that the foundation that controls all things OPC just wants it to be OPC without any higher and glorious meaning. I would assume that most of the OPC offerings you were looking at were, as you mentioned, drivers that allow connectivity from an HMI/SCADA system to a PLC platform. The important thing about it is that it is a standard that allows common manner for different types of software to connect to the hardware platforms. In theory a PLC vendor could write an OPC driver and any software package compatable with OPC would then be able to communicate with that PLC system.
There are tons of datalogging types of software out there. What is good really depends on your needs. The micrologix 1100 and 1400 actually do datalogging at the processor level and allow for a kind of batch export of the data. In my opinion pretty much useless for most applications, but its there. Same with many HMI's such as the AB Panelview. As you move up to SCADA packages, most of them have datalogging that allows the data to be stored locally on the SCADA computer or on a remote datalogging machine. They usually have datalogging files that their system can read and write to as well as the capability to send the data to a more open system such as MSSQL or Oracle databases. Then you have software such as Wonderware's Historian or Rockwell's RSSql that are dedicated data logging systems often with data analysis tools available or built in. There is also the inductive automation setup that is built around a SQL compatable database as its core.
Reasearch that stuff a little and feel free to post more questions - dont worry about my time. I post when I can.
Happy Googling!