Civ4 is crap when it comes to video card requirements.
They really just have no idea what they're doing. I have 256 Mb of video RAM, Diamond Radeon X1550 (was top of the line less than a year ago), 1 Gb of system memory, and when playing BtS on MEDIUM graphics on a STANDARD SIZE map, it crashes halfway into the game with the video memory allocation error.
THEY LIE. Whatever specs they say for video cards, I have it and it's NOT GOOD ENOUGH for those people. Words cannot express how evil, disgusting, and stupid they are at Firaxis. Scum. End of.