The unprecedented demands presented by today's hardware make it virtually impossible to get from here (older facilities) to there (a site capable of handling the requisite power, cooling, and WAN requirements) through remodeling. Yet few IT organizations have the resources to build a greenfield data center. What to do?
First, consider colocation instead of new construction. It's becoming less tenable to own and operate a data center, and those that persist in holding small facilities together with duct tape and a prayer are compromising performance and efficiency. Do a thorough cost analysis of everything required to operate what will eventually evolve into your private cloud: ventilation and cooling, power, UPS, backup generation, WAN fiber, data carriers, physical security. You may be surprised by just how expensive a do-it-yourself operation has become.
Second, develop a hybrid cloud strategy. Investigate the various software stacks, like OpenStack, CloudStack, vCloud, and Eucalyptus, with the goal of adopting a standard platform. For many, this will be based on VMware, but with hypervisors becoming a commodity, don't discount less-expensive and more open alternatives, particularly if you don't have a big investment in VMware's management stack. Then, let your private cloud strategy drive equipment standards. Some cloud systems, like OpenStack, are amenable to standalone servers with local storage, while others, like vCloud, may be easier and more efficient to implement on prepackaged hardware bundles like VCE Vblock, HP Matrix, or IBM SmartCloud. But beware of lock-in from single-vendor bundles, particularly those based on blade chassis, since you'll be stuck with the same vendor for at least two server generations.
The bottom line of any data center strategy is meeting new business application demands as inexpensively and quickly as possible. That means sensible standardization; efficiency through technology like virtualization, automation, and denser and faster hardware; use of public cloud and colocation services; and vendor selection and management that minimizes service and support overhead and lets vendors do more of your technology integration.