It is not clear if you are doing something in your home or at a business location.
Why not just rent the service you need? Is this a long term project?
Do you have any equipment you can use for a test?
Is this for use in a Local Ara Network?
Is this for use in a school?
Is this for video streaming over the internet?
Some 50 full length videos would be about 50 GB of disk space. An 80 GB drive should be enough for the job. I suppose you are using AVI format or MP4.
Memory: Yes, 512 MB might work, but better to go with 2 GB.
A single core Pentium or Celeron CPU over 1 GHZ can stream about 4 feeds at once over a Ethernet connection.