Would just like to confirm that the driver link above also works for the ATI 5670 video card that came with my mid 2010 27' iMac (i3).
I tried installing the official 5670 driver from the ATI site (verion 11.10) and I got the error "not compatible with your operating system" (or similar worded error) - even though I had the appropriate Windows 7 64-bit version. I used the link above and instaled manaully through 'Device Manager' although the drivers are for a 6000 series card!
Now I can open BF3 without that error asking to upgrade to version 10.7+. I cant notice any considerable improvement in the FPS but I haven't tested thoroughly yet. No convinced that this is the best way to upgrade my 5670 but i certainly works.
Hope this helps the OP or anyone else trying up upgrade the ATI 5670 to a version later than 11.1.