Which program taxing on a computer to run - CS5.5 Premiere or After Effects....
Also - unrelated and perhaps this should be posted elsewhere - when running CS5.5 can a Premiere or After Effects project started on a Mac - be opened and finished on a PC, and vice versa?
Well Premiere and After Effects have their own indivdual requirements. And also it depends on what media you are primarily using and what you have to do to the media. But I would guess Premiere is the more demanding. With Premiere first most important is the CPU/clock speed, next most important would be RAM and the correct nVidia CUDA graphicss card, the third important feature is a good disk setup.
Generally I would guess (I have not done anything like this myself) opening projects between systems would be yes, but there are a few things missing on the Mac version that are available in the PC version
This video walks through the system requirements for both Premiere Pro and After Effects and explains the importance of various components. Very roughly speaking, Premiere Pro makes more demands on the graphics card and hard disks (and buses), whereas After Effects makes more demands on RAM.
Regarding cross-platform work, see these pages:
The gist is that most things work, but there are a few caveats that are rather easy to take care of.