I'm pretty sure most download services will require at least an on-install activation, but I guess impulse would be the lease intrusive. You activate upon installation, and run at your leisure without the front end program.
A quick google search shows that apparently the physical version of Civ IV has no DRM
Yep. If you want true DRM-free without even a downloader/installer like Impulse, buy the latest physical disc version that says "DRM Free" right on the box.