Hi Omar

First, the ISP have to give you a MODEM that its the resposible to "transform" the analog signal(cable with RJ11 connector) to digital signal(cable with RJ 45 connector), then you have to plug a patch cord throw between the modem and router that you want to buy.

Before you buy a router, You have to test your needs, for example: " What I want to do?" R: I just want to connect a PC to the internet. OK, plug the patch cord between the modem and the PC and ISP will give you a public IP by DHCP, you dont have the NEED to buy a Router.

Ok, But you have buy a router to improve more security, have a wireless LAN and connect some PCs in my Home, I recommend you to buy a Linksys WRT54G Series. Note: The linksys cannot act like a Peer VPN, just passthrough.

Ok, But you have to implement a VPN solution throw ADLS in your business to conect to your othe business in the central park, I recommend you to buy a Router Cisco 805, its especially design to act like a peer VPN to other Router Cisco 805 peer VPN and its easy to configure.

And now if you just want to connect 2PCs through VPN(ADSL) I recommend you: LogMeIn Hamachi.. its Free

Saludos