i can run civ5 (with win7 64 and ati card)

[Davir]

if you have a folde with this here \Steam\steamapps\common\amd driver updater, vista and 7, 64 bit\Bin64

so try a run setup there from

but you need to do that every time you start or restart you pc

but it works for me

 

[shamsael]

I have Win7 64-bit and ATI onboard video.

I didn't have to do any of that stuff, it just installed and ran.