THis is the client software correct, or do we still need to have an OS on the
workstation for users
Dev
The two types of license that you would require are Device and Client Access.
Device licenses are installed on a machine to legalise that particular machine. In this case your server would require 1 x Server license (Premium or Standard) and each client computer would also require a device license (WinXP, Win7 etc). If you buy an OEM client device then it already has a license, e.g. HP laptop.
The CAL, Client Access License, is required for every connection made to the server, from a client computer, licensed per device. Assuming your SBS server will be doing authentication, then each PC would require a CAL.
So, assuming you have 10 PCs and 1 server, for SBS you would require:
1 x server license (Premium or Standard)
10 x OS licenses (WinXP, Win7, although these may already be present)
10 x SBS CALs