Increasing WLAN size

I currently have 1 WAP connected to my LAN and my broadband box also has a WAP built into it. The building I work in has thick sandstone walls and the WAP is at one end of the building. The signal degrades very badly when I get to the other end of the building and as such the speed drops off significantly.

What I want to do is add another 2 of the same WAPs to the LAN so that other wirelesss laptops can simply connect to them wherever they are in the office. Is it simply a case of configuring the WAPs to the same channel and setting them up so that they operate on the same network?

I want to be able to take my laptop from one end of the building to the other without needing to connect to each WAP individually. Simply taking it to the other end of the office should auto detect the WAP in question and allow me to continue working, seamlessly.

I have a new SBS 2003 server in place which I was thinking of making my RADIUS Server so if this has any bearing then I can use it for authenication protocols. The WAP I currently have is a Belkin F5D7130uk 802.11g enabled WAP.