You can download either the Workstation, PVF, or both. They both use the same compilers, the difference being PVF uses a Visual Studio IDE while the Workstation is a command line interface. The Workstation also includes the C and C++ compilers while PVF is Fortran only. As an aside, purchase of the Workstation product also includes a license for PVF.
If you do install both, installation order doesn’t matter. Though, you will need to install the Windows 8 SDK before installing any PGI product in Win8. Please see Chapter 6 of the PGI Installation Guide for details (https://www.pgroup.com/doc/pgiinstall.pdf)
We ship our own CUDA tools that will not interfere with your currently installed CUDA 5.0 installation, hence you do not need to remove the installation.
During installation of the PGI Workstation, you can decline installation of the pre-package Cygwin and instead use you’re own version. The Cygwin we ship is only for the convenience to customers but not required. You will need to set-up your environment but this is normal.
As for Docs, I’d start here: http://www.pgroup.com/resources/cudafortran.htm and see the “additional resources” links towards the bottom of the page. There’s also a great book CUDA Fortran For Scientist and Engineers if you want to a more in-depth understanding.
Hope this helps,