For SOA system architecture, I use the SOACP Visio stencil. It provides the symbols that are used in Thomas Erl's SOA book series.
I use the Visio Network and Database stencils to model most other requirements.
Beautiful set of stencils from Microsoft here.
There should be templates already included in Visio 2007 for software architecture but you might want to check out Visio 2007 templates.
Source: Stackoverflow.com