Here is the current
Software Compatibility list for SI5.
Specific answers to your questions as of 4-13-2011:
1. SI5.5 is
NOT compatible with any 64-bit version of AutoCAD or Visio.
This link shows a possible work around to get the 32-bit version of AutoCAD to install on a 64-bit machine. Also, keep in mind that you can't install the 32-bit version of Visio on a machine that is running the 64-bit version of other Office products.
2. AutoCAD 2012 32-bit is not supported yet, testing is underway.
3. SI5.5 works find on 64-bit operating systems. The only issues are with the integration to 64-bit applications as mentioned above.