The questions you have asked are so broad in concept it would take a couple of book chapters to answer them. To bring them into a more manageable form I suggest that you answer the questions asked by Jake and then maybe we can provide some direction towards suggestions that may help you.
I have not failed. I've just found 10,000 ways that won't work" --Thomas Edison
You may want to derive requirements from your test results. That is, you may need to find out the amount of bandwidth, physical/virtual memory, and cpu speed that your web app requires. Find the points of failure, and where exceptable performance begins. For example: Does it work on a 28.8 modem? At the risk of getting off topic, check to see if the server runs on a dual cpu. :-)
To test the content, identify each type. Get large and small samples of each type. If your content includes *.ppt files, you may need many samples. Perform all supported operations on each sample.