In your case network bandwidth and i/o will be your current bottle necks.
If you can run something like zfs, then you can use a large SSD as a dynamic cache between physical drivers and memory.
Zfs will manage this cache. If you need better performance from the SSD, just stripe them together for higher performance.
If you don't have zfs, think about identifying 'hot' files on your cluster, and move them on to local SSD's or a dedicated ram disk. That way you can use a slow san to server files, and popular files will be served from fast SSD/RAM. Think of a SSD as extending your cache. It's not a fast
as ram, but it's a lot faster than the faster than disk. (lower energy too!).
Max out the physical ram on the servers you have, this will improve caching. If you going
down the PC route, I've seen some motherboards with capacity greater than 128GB ! :-).
If you keep growing, then you will soon outgrow your data centre's IP transit. If your users
are geographically dispersed then think about putting the infrastructure closer to the users.
i.e European users get directed to the data centre in Europe..
Also look at using a optimised web server for streaming the files, http://nginx.org/ springs to mind. You can still keep your existing application servers, but direct users to dedicate video streaming servers.
Memcache is interesting , depending on your works loads this can also help.
as you grow, bottle necks will appear within the infrastructure, make sure you have good performance/error/security log's. You'll be able to capacity plan much better if know what you infrastructure is doing under what users loads. Trend this data over a period of time and you'll be able to show management why you need the new hardware x to solve bottle neck y.
If don't have the in house skills to do all of this, then have a look a content Delivery network, Their infrastructure is tuned to do this. You just pay to use it!
Once you upgrade server to multiple 10Gbit Ethernet, you'll find you run into other bottlenecks, the actual system bus of the machine :-).