It depends on your budget. Buy a switch with enough ports for future expansion. For example, if you have 10 PC's, buy a single 16 port switch instead of 2 separate 8 port and 4 port switches. Managed switches are way more expensive but it gives you more control of the network (you can create VLAN's, turn ports on/off...). Any major brand will be OK (HP, Cisco/Linksys, DLink, Netgear...)
No, you don't need to buy LAN cards for the computers if they already have an onboard NIC (it's hard to find one without an onboard NIC these days).
If you don't have a server, you will need a router to provide DHCP service.