My PC that's 4 years old with a cheapy GeForce graphics card plays CIV fine, apart from the increased time it takes at the end of the turn for the AI to do the other civs' moves.
Nowadays you couldn't find a machine that wouldn't run CIV in the shops - although at the very bottom end you'd probably want to have at least 1Gb of RAM.
And anyway, the whole point of a graphics card is that most of the 3D stuff is done in that, not in the CPU. Going back to 2D wouldn't make that much difference.