The best hardware requirements won't include a particular piece of hardware or suggest a specific vendor unless you ask. Instead, they'll spell out performance guidelines in terms such as average transactions per second per active user, maximum acceptable storage latency, and general capacity requirements based on your size and other application-specific variables.
If you can get these kinds of specifications, you'll have the information you need to intelligently scale your existing infrastructure out to meet the long-term needs of the application. In these cases, you can generally assume that your vendor has done rigorous scalability testing and knows how the application will react under increasing load -- all signs that you're in good hands.
Standard operating procedure
Run-of-the-mill hardware requirements tend to be both too specific and too general -- and result in wasted capacity and unnecessarily high costs.
Such specs are born of ignorance rather than malice. If you are able to show the vendor that you have done the research and understand what you're doing, it will be willing to support you in configurations that color outside of the lines. In the cases where the vendor sticks to its guns, there isn't much you can do, other than get the executive sponsor in your organization to try some arm-twisting. The best you can do is get involved in the process early, because that's where you can have the most impact on the form the final implementation will take.
Here's where things get ugly: The software comes shipped to you in a 42U cabinet full of server and storage hardware. Yes, the most difficult vendors to deal with are those that insist on bundling their software with hardware.
Unless the vendor has real hardware competency and a demonstrable need to deliver its solution in this way, seldom will your organization's interests be served. You see closed, fully integrated solutions like this more frequently in the health care sector, where support requirements are understandably more stringent. Even in those cases, it makes sense to scrutinize what the vendor provides to see if you can deliver the same services by leveraging your existing infrastructure. Such integrated solutions are a thorn in the side of the IT personnel who have to work around them and should be avoided wherever possible.
The most effective way to keep your storage strategy on track is to communicate regularly with the highest levels of your organization. Make sure to sell the value of adapting current infrastructure to accommodate new applications and make clear that you want to be involved early as new applications are considered. And if you're already supporting multiple storage systems, don't despair. Start laying the groundwork for integrating your storage platforms during the next hardware replacement cycle. Primary storage is rapidly approaching commodity status, and it will be easier and easier to make this argument with stubborn software vendors in the future.