Psyringe
Scout
Definitely XP. Among its other problems (like lack of drivers), Vista is a memory hog, and the amount of free memory is one of the most important factors that influence Civ4's performance.
Actually, I think the best OS for Civ4 is Windows 2000, because Win2000 has an even smaller memory footprint than XP.
Actually, I think the best OS for Civ4 is Windows 2000, because Win2000 has an even smaller memory footprint than XP.

. Paste a copy of the Warlords shortcut in the desktop, click with the right button of the mouse in the shortcut and in the part of destination add this: mod="Mods\Rhye's and Fall of Civilization". With the quotation marks.
Viva il Rhye.