System Reqs for Civ 4:

Minimum Requirements:
1.2GHz
256MB RAM
64MB 3D accelerator
CD-ROM
1.7GB free disk space

Recommended Requirements:
1.8GHz
512MB RAM
128MB 3D accelerator

Running games with the bare minimum requirements generally sucks. Espcially if you are using shared video memory.

Civ 4 doesn't require too much power, but your system isn't all that powerful, esp. when it comes to video processing.