As a rule of thumb, you need 1 processing core per stream of HD video (2 SD streams = 1 HD stream). So if maximum streaming would be 4 HD streams, you should use a quad core processor. I have utilized both AMD and Intel chipsets for my builds. You also would want to go with 1.5GB RAM per stream + 2GB RAM for the OS. 4 HD streams, you should use 8GB RAM.
For storage, when you get close to the 4HD streams, you want to make sure to get SATA III drives at 7200RPM or better - size is dependent upon how many videos you want stored on the system - I find that 2TB (without DVD/Blue-RAY storage) is plenty of space.
Here is a good sample build:
http://pcpartpicker.com/p/2sGeA
Other things to think about - Windows 7/8 doesn't natively...