You can use a 2620 with beefed up memory, but I have had some high CPU utilization issues. They have smaller 1700 series routers but I am not sure about the modules supported. I switched it out with a 2651xm (no CPU issues at all) you may want to go with a little bigger router like the 2651xm. You will also need a PRI card, this is big bucks, about $5000 depending on your discount.
Good Luck,
Geoff