Hi, I hope that is okay to ask, as I also have a Radeon 5870 HD 1gig card in a mac 5.1 and while it has been great and still is, never had any problems with it. I have been wondering for a while to upgrade the card, looking for more power. I do use 3D modelling programs (though not at the moment) such as Blender and create basic 3D models. It is a learning process as well. And I also play games, though not as much. I noticed a few times on a complex model the textures in Blender would make updating the screen run very slow, it was unworkable at one point. I had to turn off the texture move to where I needed then turn on the texture. I figured if I am having problems with this model just think on something much more complex.
I am not sure why this was happening but I guess it is the card I have. From what I remember Blender does not natively support Radeon cards but supports Nvidia cards I am not sure if that is correct please correct me if I am wrong.
So yeah, if I can ask what would be a good upgrade/options? from the 5870HD I connect to 3 screens I think there is Radeon 6970HD but as stated Blender does not support Radeon cards for processing as far as I know, maybe it is different now.
If I was to update I would look at the 6970HD, as the 5870HD card has worked and I never had a problem but I am not sure, just like to know what are the options one can take.
Thanks