The problem with "environmental mapping" is it takes a lot or horsepower to generate all six of the views, on the fly, and then project them onto the surface. That has to be done every single fram when doing real-time environment mapping.
Now, when you add anti-aliasing, then you can easily quadruple the amount of memory that one function needs. Now add the anti-aliasing passes needed for each texture and it becomes a real performance bottleneck as well as a huge resource hog.
The shadow texture size suffers some of that same thing when anti-aliasing is applied.
Just experiment to get the best tradeoff in visual and performance you want.
No,..adding another video card does not help anything. There is really nothing you can do. The only time I find "environmental mapping" nice to use is when I am working on films and have a lot of exterior shots of shiny planes.
