I'm using a cisco 2600 as the VPN gateway. I have 15 IPIP (GRE) tunnels to various locations. (Guam, Fiji, Curacao, Samoa, USVI and some domestic)
The remote sites are either 1600 or 2600 routers. These remotes have several different types of connectivity. (WDSL, ADSL, SDSL, FRAME, 56k PPP) All sites are over the internet. I am also doing some VOIP through the tunnels as well.
I haven't noticed any performance issues as of yet. All remotes support 4to6 terminals and 2to3 print servers (HP Jetdirect).
On the ethernet side of the remotes I use private addressing and I use NAT for internet access.
I hope this helps