The answer depends upon whether you will use 3des to encrypt new traffic that currently does not flow over your existing link, such as bringing up a new remote site location. If the encrypted traffic is new, that is something additional that does not affect current flow, then you will need to analyze the traffic pattern.
I believe that IPsec will add approx. 50 to 80 bytes to each packet, depending upon whether ah will be used as well as esp, if GRE will be used, and if tunnel mode (new ip headers) will be used too. (Add 24 bytes for AH, 24 bytes for GRE, and 20 bytes for new IP header).
If the IPSec vpn will only be used for existing traffic flows, instead of new ones, the link util should not increase that much; it is CPU intensive more than bandwidth, and I see that you have offloading encrypt cards.
Let me know if you need anything else.
