For sequential reads (like video streaming), the most important factors are the number of spindles and the number of bottlenecks in the data path. Each drive adds its performance (in terms of MB/s) to the aggregate performance of the raid. The speed of each drive will be a factor, but usually even just a few drives is enough to saturate some other bottleneck in the data path.
5400 and 7200 RPM drives (and even 10 or 15k RPM) all perform pretty close. The first three pages of these benchmarks from tom's hardware will demonstrate that the factors other than spindle speed are actually more critical from a throughput standpoint.
Firstly, the disk controller has to be able to read from a raid 5 as fast as your clients can request data. This means typically it needs to have a decent amount of internal and external bandwidth, with as few shared resources as possible. Secondly, the connection between the NAS and the network the clients will be using to request data needs to be thick enough to handle a bunch of concurrent streaming. Third, that same network can't have under-powered ASICs powering the ethernet ports, leading to an "oversubscription" environment where any port can burst to 1Gb/s, but they can't all do it at the same time.