Its possible the memory changes in V1.52 (extensive, and put more work on the video card to save RAM) have had an effect. Try this change in the ini file
; Allows some memory savings *** ALT-TAB WILL NO LONGER FUNCTION *** (0 stops the memsaver, 1 enables it. But set to 1 willload grahics card more. If you have graphics processing / capacity issues setit to 0 so that the card (or onboard graphics chip) has less load
MemSaver = 0
In your case setting it to 1 may work as this makes more memory available in main System RAM - but a setting of 1 will place more work on your video card
C:\Documents and Settings\New User\My Documents\My Games\Sid Meier's Civilization 4\CivilizationIV.ini
Regards
Zy