Network not shareing well

I had three homebilt desktop computers and my laptop sharing a dial-up connection through an SMC Barricade (7008ABR) router all running Windows XP Home Edition SP3. Everything worked fairly well, of course dial-up is pretty slow, especially when my wife and I were both on the internet at the same time. It was like there was a traffic cop directing so that when I clicked on a site my wife would have to wait till my site come up then she got what she wanted and vise versa. Then a storm came through called Ike and I had to replace two of my desktops with used Dell's running XP Pro and replaced my 8-port Barricade with a 4-port Barricade. At first everything seemed to be OK. The machines saw each other, shared files, shared the printer and got on the internet. Then I discovered one problem - they are not truely sharing the internet. While one machine is on a site the others are basically locked out. When another machine makes a request, it just sits there for a while and then the page can't be found screen comes back. So only one of my machines can be on the internet at a time. Have I overlooked a setting? Is it something about the Dells or XP Pro or router? What's going on? Internet searches seem to be much slower too.