Your computer is pretty much the same as my old computer which I gave up trying to play Civ 4 on as it was so painfully slow. I'm afraid that there is little to do to speed it up. You can try removing the check next to show friendly moves and show enemy moves, but that means that you don't see what the other Civs are doing, just the end result. Single unit graphic will help as will turning all graphic settings to minimum, but you have probably done that already. You may find it easier running in Windowed rather than full screen mode, particularly if you want to switch between applications, you'll have to see whether it helps in your case though.