Thanks for posting.
By design, FileMaker Server 9 is created with a better performance output than FileMaker Server 5.5. Also, just the upgrade of the server machine and its specs may help out. If you have machines that can support FileMaker Server 9 and FileMaker Pro 9, I would recommend downloading our free trial software and giving it a test run. You can download the trials under the Trial Software section here:
The .fp5 files will convert to .fp7 but the .fp7 files will not overwrite the original files. That means that you can do a test run of a FileMaker 9 environment and not have to worry about damaging any of your databases.
That said, I believe the necessity of the upgrade is relative to the user. If you're happy with your solution and everything is working well (for the most part) do you really need to spend for an upgrade? I would start by looking at your solution and seeing if there is anything you can do with it to increase performance. Does a compact copy help? Are there any fields that do not need to be indexed? Can you cut down on unstored calculations? Are there any dead file references?
I would definitely 'try before you buy' and work on the solution first. Hope this helps.
What are hardware requirements for FM Server 9?
You can find the system requirements for FileMaker Server 9 in Knowledge Base article 6428 (Support > Knowledge Base).
Here is the link to the Knowledge Base: