Just as important as products is close cooperation between your IT and legal departments. IT must help legal understand concepts such as metadata, archives, and tiered storage so the lawyers can more accurately describe to IT the scope of a discovery request.
If IT and legal aren't on the same page, the lawyers may find themselves in trouble if they present an incomplete set of data to opposing counsel and judges.
IT also can expect some education. For instance, it will have to develop processes for legal holds, in which messages or files that fall under the scope of litigation must be stored in such a way that they can't be changed. Users that have access to information under legal hold have to be notified -- and prevented from accessing and changing that information. IT also may have to be brought up to speed on legal requirements for electronic evidence.
Of course, IT may find its relationship to be a friendship with benefits: the money for e-discovery hardware and software often comes from legal budgets. And the policy and processes that define an e-discovery system can form the groundwork of an information life cycle management (ILM) strategy, resulting in further efficiencies in storage management.