NAS devices come in many varieties depending on if you build it yourself or purchase a vendor product. The speed is typically one gigabit Ethernet connection but this can be changed to multiple gigabit, 10 gigabit, fiber optic by adding a pci-e network card(s). Older parts can be used which may be limited to 10/100 megabit. If you need an exact answer for speed, simply look at the wiki on gigabit.
The capacity range again varies, people have built 40 TB (terabyte) machines and other just have 2TB. With port replication and addon hard drive controller cards there is hardly a limit on size. A board with 6 sata ports can be replicated (1 to 5 port) allowing for 30 drives to be attached, if 3TB drives were used in raid 50 that would be 72TB of storage.
As far as fault tolerance, raid 50 is fairly stable if set up correctly, raid 10 has been touted as one of the best setups since more drives can fail at one time without data loss. All of those features can be used on typical NAS devices.
Here is a typical management feature youtube rundown of FreeNAS 7, it covers the basic controls through a webpage.
http://www.youtube.com/watch?v=5S8ixAR4Opo