ARCHICAD 6.5 has been released in 1999, as the flagship product of Graphisoft.
The major features themes of ARCHICAD 6.5 include: ARCHICAD versions
ARCHICAD 6.5 System Requirements
ARCHICAD 6.5 version's OS, CPU, RAM, Video Card, etc. requirements: http://www.graphisoft.com/support/ARCHICAD/system_requirements/archicad65.html
How to run ARCHICAD 6.5 parallel with ARCHICAD 7.0 or 8.1 versions
You only need to copy the ARCHICAD 6.5 that's in the Support folder of your ARCHICAD7.0 or ARCHICAD8.1 Install CD (copy the whole folder not just the application) onto your HD. This special version of 6.5 runs with the WIBU key provided with later versions (it doesn't run with the original 6.5 keys nor does it run with a WIBU PPU dongle).
Converting ARCHICAD 6 files
From Graphisoft page you can download old ARCHICAD packages which are pre-installed and updated to the latest available build. You can use these packages to convert your old ARCHICAD files to a format which is supported by current ARCHICAD versions: http://www.graphisoft.com/support/archicad/downloads/Fileconverter.html