The best way to assess how a server will respond with your application is to benchmark it with your application. Unfortunately this is not quite as easy as more arbitrary benchmarks, but it does give you much more realistic view of expected performance.
Exactly how you do this depends on your application, but the general case is:
- take a copy of your live data
- run a set of common queries to test baseline response times
- run said queries multiple times concurrently to mimic a few concurrent concurrent users
- keep increasing the level of concurrency until responses get to slow or things break (time out errors, out of memory situations, and so on)
This gives you an indication of how your application will scale on the given hardware, you can say things like "on this hardware we can serve X concurrent users with response times below Y seconds". For this to be accurate you need to design the set of test queries such that it represents the usual balance of activity. This is often done by analysing what users usually do (base user logs in, gets list of tasks due, opens first task, fills in form, opens second task, fills in form, logs off) and scripting that process up so you can replay these stories many times for may users (the many users bit is important - if you use the same user and access the same few data rows in each test you will see unrealisticly high performance as everything will be running from cache where in more real circumstances disk access would be needed).
Getting this "perfect" can be bit of an art form - it requires good knowledge of how your users interact with the application. Some of our clients outsource this sort of testing to a specialist third party company when we deliver new versions of our applications, to ensure that our setup keeps our scalability and response time promises, though if you have resources to spare it can be done in-house, and is a good idea to do from time to time anyway as it can highlight performance problems introduced by new features before you put the changes in front of your users.
Search for "non functional testing" techniques, specifically "stress testing", "load testing" and "scalability testing", for more info - there are tools out there that can help automate it all, though the technique/software that is best for your app will depend greatly on what your app actually is.