Hi, to answer this fully we need to know how the new circuit is going to be terminated. Is this a ‘true’ leased line, or bandwidth over Ethernet ?
If it is really a leased line, then you could use the FlexWAN modules on the 6500. This is essentially a Cisco 7200 router on a blade for the 6500, and you plug in the modules from a 72007500 to provide whatever interfaces you need. I warn you, this is not a cheap solution !
I would suggest that you are probably better getting a separate router, or modules for the existing one (if it has the necessary performance, and interface type). I think that it will prove to be far more cost effective.
If you post more details of the interface type for the new leased line, and your existing router, then maybe we can propose another solution.