Unfortunately the vanilla base game has been pulled in favor of Trinity. You may still be able to find just the base game available at retail, which can then be registered within Impulse for download/updates.
So wait- if you buy a retail copy of the game, you can register it on Impulse and have it as a downloadable title? Or did you just mean the updates?
I'm looking at 4-pack of games (GalCiv 2 Ultimate, Darkstar One, SoaSE, and some other game) for $24 and it's probably far more economical for me to go that route if I can just download it. I'd normally want the case/manual for my games as part of a collection, but with the PC, I'm starting to see the wisdom of DD.