Anything web I like to start with a view of the stats when considering performance. So if you're talking about streaming media to the web you need to determine, first, what the baselines are that you are at right now.
How much data is being sent daily, monthly, hourly, what are the peaks and valleys, how many concurrent users must it sustain, what shoudl the MTBF be.
Then, you need to consider the growth. So if you have access to historical statistical data, you should be able to draw a general growth curve. Keep in mind that web traffic generally grows exponentially. Espectially for sites that are viral (assuming you are talking about a viral-type site).
Based on this, you should be able to begin your testing and then tell whether you are working with a sustainable system or not.
Note, this is just testing within the happy bubble that is "everything being equal". Remember that you may encounter malicious attacks, server backups, etc. while this server is operations, that will also eat up resources and need to be accounted for in your testing.