The hardware requirements for FreeNAS are really minimal. You actually do not need the fastest hardware to run it. A Pentium class (a Pentium 100 maybe) processor or higher, with 100 Megabytes (yes Megabytes) of RAM, the capability to boot from CD/DVD/USB and about 100 Megabyte of hard disk space is all you need to install the FreeNAS system.
From this point on, the main piece of hardware you will need is hard disk storage. Because the computer will be hosting your files, you will need a hard drive that can accommodate your current and anticipated future files. If you do not have compatibility problems, go with the largest drive that you can connect to your computer.
The last item you need is a network connection. You can configure the computer to run on your wired network, or fold your sleeves and make it a wireless one. The wired one is easy but will involve cabling to some extent, while wireless configuration is hard but has the benefit of requiring no cables. The decision is up to you. In this article, I will go with the wired option.
The wireless option needs a little bit of router configuration (of course an additional wireless router purchase) and the 802.11 g standard allows maximum 54 Megabits per second transfer, which will not be sufficient for High Definition media streaming. If you still prefer to go this way, FreeNAS supports all the wireless adapters supported by FreeBSD. Just have a look at the FreeBSD's hardware compatibility list.
Read on for installation and configuration.