Uninstalling 6.5 left the Program folder with Matrox plug-ins and Project Presets intact. Either clean them out or Uninstall 6.5 again, then delete the Program folder before reinstalling.
Probably the best card for 6.5 is/was Canopus Storm 2. There are probably dozens in the dust bin that can be had for cheap. Maybe Steve Gotz can pry his loose from under the door and ship it to you.