Hi

You need to freeze on the kinda vpn features you want in you router like static peer to peer vpn , static supporting dynamic vpn clients , encryption strength etc.,.

In the recent past the feature sets have been well defined and you can choose the feature set which can support your requirement which you will like to have in your router..

In this case you need to have advanced security feature set to have vpn features configured in your box.

http://www.cisco.com/en/US/products/sw/iosswrel/ps5460/index.html

You can also refer the feature navigator tool which provides enough info on the hardware u require and the ios file which would be compatible with your current hardware config...

http://tools.cisco.com/ITDIT/CFN/jsp/index.jsp

regds