I would have to recommend that you look at OSI's PI.
I have had a great deal of success using OSI's Plant Information (PI) software. At my previous job, working for an electric and water utility, we successfully integrated 5 steam units, 3 combustion turbines, 26 water pumping stations, 700 waste water lift stations, and a GE/Harris energy management system using PI. The GE/Harris system is slated to go into production later this summer, at this time the interface is running on a test server.
The control systems we integrated were from Bailey, Woodward Governor, Siemens, Square D, and GE Harris. A few Allen Bradley systems are in the works.
All of the historical data is archived on a central NT server (about 60,000 tags once the Harris is completely integrated). The system is very reliable and provides information to any user on the company's intranet. OSI has a client package for building HMI type screens and a data link into Excel. I also like to use the ODBC interface to build web pages for reporting.
The systems have been integrated over time but we have been able to keep all of the historical data ever collected (since 1997) online because of the data compression algorithms OSI uses. The data is stored in a RAID array on the NT server. The volume of data has risen to about 1GB every 27-30 days. With 18GB drives available for RAID array now, that is about 1 drive a year.
You should take a look at Rapid Historian from Automsoft (www.automsoft.com) - we've used it on several applications and have found it very flexible and easy to configure. With OPC and SQL/ODBC it's pretty open, and as it's built on an Object Oriented database it is extremely fast.
I spent 8 years in the Information Systems end of the spectrum as a Network Engineer. I had the opportunity to work with several VERY LARGE database systems ranging from Oracle, Sybase to MS Sql. In All these encounters I found some very impressive configurations performance specs.
Since I crossed over into Industrial Automation, I have had the opportunity to utilize several of the *real-time* databases on the market. The first time I saw Wonderware's Industrial SQL server operate, my jaw hit the floor. Many of their competitors have claimed that they have the same or better performance, but I have yet to see anything even close. I know of installs that achieve 30 analog point @ 18ms resolution, Many installs that handle 200 - 300 points @ 100ms resolution or better. This thing Smokes! IMO.