Can you give more specifics on the symptoms you are getting. It would seem you start ok, even with Large maps - but you dont say what happens then. eg in late games circa 1800s with lots of civs end industrial age/start modern:
slow turn times, slow game play, large save files, save games in mins not seconds, movie stutter, etc
There were some issues with some original CDs (they got the play disc label the wrong way round) that sparked off copy protection rumours - still maybe true to a degree, but its not the main issues with the beast. It could still be driver/pagefilefile/ini setting related, but I have a suspicion its the memory issue in your case. It would help to see your Dxdiag dumps and the additional symptoms before ripping the installation apart. Also, if you can run it in window mode, then in late game play post the memory use in Task Manager for civ.exe
(Start button - click -choose run - type dxdiag - press enter - let it chunter away for a min - select "save all information" - then post here using the 'advance' button below)
Regards
Zy