This is sad that a game is giving all this grief. You should be able to play this darn game with the specs you have on that Dim 2400.
Silly Sid and
Gang should come out with a patch for all HIS loyal Civilization users.
Now a regular PCI slot card should do the trick for you pretty easily. On any other game I would suggest the (I hate to say it) Nvidia 5700 PCI...only because there is not allot of options for PCI cards. But then again people are having trouble with that series...so it baffles me.
Instead of me trying to take a "stab" at it....I will suggest the Dell Community Forums. There are better minds then
I there. Register there and take a peak. First Search...some of the
Gurus there are a little stuffed shirted about people that do not search first. (They get real tired of the same question over and over: My Dim 2400 won't play games...what AGP card do I buy?"

) I am not saying you are asking that. Also many of these
Gurus are pretty good at all types of computers. Dell is pretty open to anyone asking just about any type computer set-up.
When it comes to Dell systems though...these
Gurus are the best and will have some good ideas. I have learned allot there. I have three Dell systems and they have helped me with mine over the years. There is one guy that "I" think is the brightest. His nick is: "ejn". He has been around for years. He is lightyears past anything I would know.
Hope this helps some.