Setting Up A VPN on CISCO 2600 / 2500 / PIX520?

Haakon asks: "I'm considering setting up a VPN across our USA offices by upgrading the existing 2600 / 2500 routers to FW / VPN feature set. My question relates to some 'Cisco' advice passed on by our supplier: to establish a VPN link between the USA and an existing PIX520 in Europe apparently we have to use another PIX520 in the USA -- Why is this?"