What type of architecture discipline
is used to specify all the software
systems and their interaction together
for a whole organization?
Enterprise Architect: usually via an Enterprise Architectural Framework like TOGAF or Zachman.
This work might also include involvement of other architects from specific disiplines - such as Data Architects.
... overall architecture as we move to
a new data/hosting center ... is
Enterprise architecture or software
architecture used for this kind of
planing?
Not an Enterprise Architect. The actual role might depend on the company but I'd expect it to be either a Solution Architect or an Infrastructure Architect. Movement of infrastructure such as you've descibed would most likely be done by an Infrastructure Architect, might might be part of a wider programme of work that is lead by (or involves) a Solution Architect (perhaps as part of a specific project).
What is the difference?
Firstly you need to appreciate both "vertical" differences in terms of abstraction / detail, and "horizontal" differences in terms of subject area.
- Enterprise Architects aren't focused on a specific domain (some would say they cover everything, others might say they are so high level they don't cover any).
- Likewise, Solution Architects aren't focused on a specific domain; they work on actual projects with specific business goals and requirements. I'd expect a Solution Architect to have a broad understanding of multiple "subjects" but not necessarily deep understanding.
- Other kinds of Architect (Data, Security, Infrastructure, etc) are different; they have a very deep understanding of a specific subject area and work at all levels, including contribution to the Enterprise Architecture, standards relating to their subject, and supporting specific projects where necessary.
A Software Architect is often seen by many people as being the same as a Solution Architect - but I'd only agree to a point.
- To me, Software Architect (as the name suggests) focuses on software, where as the term Solution is more wide ranging.
- In some ways a Software Architect is a subject specific architect as defined above (like an Infrastructure Architect). If you were going to embark on a major bespoke (custom) software development you'd want a Software Architect to lead that as the development of bespoke software is a specialised area.
Things become murky when a singe person performs more than one architectural role - and this is quite common. I would guess that peoples perceptions of what a specific kind of architectural role is become tainted by what they percieve a specific person (that they know or work with) did; for example you might work with someone who is called a Software Architect but they might also be performing the role of a Solution Architect.