Answer
If the server's processor and memory still have headroom and the bottleneck is genuinely storage-related, expanding DASD or adding SAN attachment is usually the faster and cheaper fix. Pull current ASP disk busy statistics before assuming storage is the actual cause of a performance issue.
If storage, processor, and memory are all constrained at once, that is usually a sign the whole server has reached its practical ceiling, and a server replacement conversation makes more sense than a series of piecemeal upgrades.