How does installing XI affect an SAP release?

Our company is currently running R/3 release 4.6C. I am researching what it would take for our company to start using SAP XI. Is it possible to install XI with release 4.6C or do we first need to upgrade to SAP Web AS? Can XI be installed without other components of NetWeaver? What are the hardware requirments of XI? Does it need it's own box(es) or can it be installed on the app server box? Thanks.

XI will run as a separate instance. It is a middleware product that plugs in right between your current systems. Installing XI does not impact your SAP releases at all; it won't touch them.

So the question "does it need own boxes?" should be answered "Yes!" (actually it can run side by side on the same box as R/3, but as a separate instance, which is just like an own box).