Not sure what may be in the Civ3XInf.dat file. There could be a wealth of alterable information there, but the problem with DAT files is that in order to view what it contains, you need to know or have what was used to create it. It could be opened with anything, or contain anything.
The possibilities are endless, but if the program or editor could ever be found to edit it's contents, it could possibly provide an enormous amount of editable content, or could be something quite worthless!
That's an interesting thought. On the off chance that it might work, I ran that file through SavExpnd (to use that, run 'SavExpnd "inputfile.in" "outputfile.out"', without the single quotes but with the double quotes). And I was able to figure out what it is. To avoid entirely changing the topic of the thread Tom posted this in/threadjacking, I've started a new thread for this topic.
Civ3XInf.dat appears to be entirely in binary when looking at it in a hex editor. Disappointing, perhaps, but then again, compressed SAV games appear to be entirely binary in a hex editor, too. So on the chance that it might do something, I ran SavExpnd on Civ3XInf.dat. I ended up with a smaller file, which looked dubious, as I thought SavExpnd was supposed to expand a file, y'know! But I opened the "uncompressed" version in a hex editor, and looked for something intelligible.
What I found was a couple of Jaimo references near the beginning, several near the end, and a whole bunch of gobblydygook in between. Checking the Easter Eggs chart, I saw that the Jaimo unit appears in your units folder after you've triggered the Elvis unit. It dawned on me that this unit obviously had to be stored somewhere prior to being unlocked, and this was probably where.
So I told 7zip to open the uncompressed version as an archive, and, voila! All the files needed for the Jaimo unit! Although I can't compare it directly to the Jaimo unit as I haven't triggered Elvis since I last installed PTW/Conquests and don't feel like setting my calendar to January 8th, everything looks fine (see screenshot - the U in the filename stands for Uncompressed).
The good news is, we now know what Civ3XInf.dat does (the version in the Civ3PTW folder appears to be identical, although I haven't run this process on it). The bad news is, it doesn't contain any editable content, unless you want to change the Jaimo unit for some reason.
The same technique might work for some of the other files whose purpose is currently unknown. I wouldn't bet on it, but you never know.