Dual install is pretty easy.
1. Copy "Sid Meier's Civilization IV" in Program Files/Firaxis Games, to "Sid Meier's Civilization IV v1.52".
2. Rename "Sid Meier's Civilization IV" in My Documents/My Games, to "Sid Meier's Civilization IV v1.52".
3. Install the patch.
4. Run from one directory or the other to get the version you want. You'll also be able to install mods separately for the two versions, save games will be separate for the two versions, etc. The new version will have a clean install with no mods, which is probably what you want (since many old mods won't work).