Sorry but I would hardly call the 5870 crap.

I'm running windows 7 and upgraded from the AMD 5870 1 GB to a NVIDIA 670 4GB card. It's nice to have the extra video ram with modo and games,
but I tested the frame rate in MoI with the same file, settings and monitor resolution as in this thread:

http://moi3d.com/forum/messages.php?webtag=MOI&msg=5252.72

I got almost exactly the same results, 17 to 20 ms.

I would say that your problems have more to do with ram, you said your working with a 800 MB file. MoI is a 32 bit application so it doesn't matter if you have
28 GB of ram. I'm not sure how the Macs manage memory but on windows the best you could expect would be 4 GB of ram for MoI.