The IHE is an international standards development organization focused pretty much exclusively on sharing health information among systems. The group promotes standardsdeveloped by other organizations (like HL7) and also develops a set of its own standards, many of which are used today in healthcare environments. IHE's relevance in the current health information exchange context is that many of IHE's standards are incorporated in recommended or required standards coming out of HITSP, ONC's federal advisory committees, and technical groups working on initiatives like NHIN Exchange and NHIN Direct. As new areas are evaluated for coverage by standards recommendations stemming from meaningful use, and government health IT initiatives, IHE remains one of several possible sources of standards to be selected. (Generally speaking, ONC and groups like HITSP do not create or define standards; instead they survey the available landscape for existing standards that might be used in support of a given purpose or function, and try to choose the best alternative among those options.)
IHE contributes to interoperability by defining and/or promoting common data and service standards that, if implemented by multiple parties wanting to exchange information, facilitate health information exchange by providing common technical ground. Some IHE standards are also designed to abstract out variations in underlying technical standards or approaches adopted by healthcare organizations, to present a common representation that others can use. For example, IHE's cross-enterprise user assertion (XUA) is designed to provide a consistent representation of identification and authentication information about a user, even if the underlying authenication mechanisms or credentials used within different environments are not the same (e.g. one uses Active Directory and Windows Authentication, another uses Oracle Identity Manager, and a third uses Tivoli Access Manager). Similarly, the audit trail and node authentication (ATNA) standard doesn't require that every organziation use the same audit record format, but does specify a common format in which audit records are communicated between organizations.
In terms of how to utilize IHE for purchasing a system, you generally don't have to. Many IHE standards are already supported by EHR vendors and other health IT solution providers (even open source ones - the ONC's CONNECT gateway implements quite a few IHE standards, for instance). To the extent that vendors want to take advantage of the incentives provided to eligible providers under meaningful use, those vendors will certainly make sure that their products comply at least with whatever standards are included with the certification requirements issued by ONC (which will be tested by NIST or parties it authorizes). This doesn't mean that a given product will conform to all the IHE standards, but IHE is pretty pervasive in commercial health systems, so if you are looking at established health IT vendors, a lot of IHE is probably already in their product offerings.