I'd wait until there are fixes released for this game either by civ patches or video card driver patches. I can run everything at the highest level and 4x anti-aliasing with a ati 9600xt 128mb and it looks good to me -- when it doesn't crash to the desktop, reboot, or go all to hell... which will happen anywhere from 5 minutes to 2 hours into the game. From what I've been reading though, it doesn't look to be card specific.
But right now, this game doesn't warrent purchasing any equipment for.