12MB/s is the max you can push through a 100/t connection. If you upgrade to 1000/t (or Gigabit Ethernet) then you will have a max of 120MB/s. 100megabits/8bits to a byte = 12 Megabytes per sec, similarly 1000megabits/8bits to a byte is 120Megabytes per sec. And Fiber or 10,000megabits/8bits per byte comes to 1.2GB/s, but that isnt really for home use yet
Plus there is traffic on the network, and overhead for the transfer protocoll, so you never really get the max through the network, but you should be able to get a solid 115MB/s if everything is set up correctly.
You will need 1000/t on each computer (default on most computers made in the last 6-7 years), you will need a 1000/t router or switch, and you will need either modern HDDs, an SSD, or a RAID array (on both ends for both reading and writing) to push the data fast enough to max out the bandwidth. Also, if you are more than ~30ft away from a device then you may need CAT6 wire instead of CAT5 or CAT5e (your mileage will varry, I have cheap CAT5 in my house, and only one computer refuses to connect at 1000/t). If you have a NAS I highly suggest RAID 1, 10, or 5 (5 is a bit slow and harder to manage than the others), so that if you have a drive fail on your NAS then you will be able to recover the data that was lost. Also it will help throughput if you have multiple users pulling information at the same time. For an OS I suggest FreeNAS if you are not afraid of Linux, or Windows Home Server which is bone-head easy to set up and use.