Load testing will give you some idea of how your server will perform under certain given conditions. This is good to know before you place a server in the production mode. However ...
There are applications that your clients install that can severely limit your server's capacities. For instance, if your client is uploading images to a Gallery2 app, it creates thumbnails during the upload process. This can consume enormous amounts of cpu and slow down your server. If you have a couple of clients doing this along with other client's MySQL queries, your server could have a heart attack. While this may only constitute a relatively small amount of actual human time, the humans will notice it and give you hell about it.
Another thing to remember is that when doing load (stress) testing, you can use a lot of bandwidth. Also, never test on a production server. Test it BEFORE you put it in production.
I need to test Speed/Response time and Uptime also..
For response time, try ab. It is included in Apache, usually in the support/ dir. For uptime, you could use NAgios which is very complete or also get a 3rd party monitoring service. We are using www.hyperspin.com.