You obviously don't change cards every year (or less, like some around here
). So, I wouldn't scrimp and I'd buy a real good card. While the 6870 might be more than what you need, at the moment, I think you'll be happier in the long run. It's not like it's crazy expensive.
If you plan on buying any CUDA apps., or you already own any (highly unlikely as you don't own nVidia), then the recommendation for nVidia makes sense. If not then it's worthless to talk one brand over the other. It's not like CUDA is the only programming language out there and every app runs it. Far from it, actually. You have to hunt around to find uses for it. One program here and there, is all you find.