Get a gigabit ethernet switch. There's a variety of price points & features available - it really depends on what you mean by "needs to be fast" - how big are the files really? Are you doing video or other large files, or just standard business documents and email?
The actual connection is very easy, you will either need a crossover cable, or a switch with an autosensing port (auto mdix)
Notice that if you get a gigabit switch all the network adapters on the computers or devices that you are connecting to the switch (devices like a NAS) have to be compaitble with you gigabite switch meaning their Ethernet port has to be gigabite also. Another thing to keep in mind is that although it will increase the speed of your local network it will not add any speed to your broad band connection because that it is limited by you ISP. If the switch you buy has a port labeld with "up link" you do not need a crossover cable you can use a regular Ethernet cable preferably CAT5e or CAT6.
Even most consumer grade switchs will auto-sense crossover/strait. Just make sure you find out. If the switch does not support this VERY simple feature, I would question the quality as it's about as standard as color is for TVs.
You don't "have" to have all gig NICs, but obviously if you try to transfer a file between a 1000 nic and a 100nic, you're only going to max out at 100.
managed is nice, but not needed for a small lan. One nice thing about a managed switch is you can setup a true firewall on the port connecting to the router.