Chris, my card is actually the mobile version of that card so it's not quite as powerful, but it is still a dedicated GPU with 1GB of Ram based on a similar architecture. As you point out, however, my average fps should not be around 25-30 especially with a modern dedicated card (the card I have iis better than the ATI cards built into mac laptops, which you said gives you around 60fps). I'd expect at least the 60 you get on your mac.
For the record I have closed everything in the background and have made sure I am only running 1 copy of the game. I have also tried disabling Vsync, etc. I guess it could be the ATI drivers themselves, but if that were the case then I would think you would have had more problems reported by now. I can play around with some older drivers and see if that changes things. It's a real shame because I'd rather be playing AVWW than SC2! AI War and Tidalis both run great, by the way.
Another note: the world map averages about 15 fps as well. In my settlement I am around 40.