I want to build a dedicated home file server that will also be used for streaming numerous video files. Does the SATA bandwidth matter if I will be using 1tb 7200rpm drives? Also, would I be able to downgrade the drive in any way without noticeable performance hits (disk speed)?
The SATA bandwidth does not matter.
Even relative old SATA-II (3.0 gigabit/sec) is able to push 270 MB/sec.
The only exceptions to this are:
Streaming video files does not require a high RPM disk since the data is usually read sequentially. There is no advantage in using 7200 RPM drives for this. A 5400 RPM drive will work just fine, and might use less power and generate less heat.