my experience with VPN accelerators is quite limited (VPN1411 on Alix 2.D3), but maybe you can avoid making the same mistake as I did. This card decreased the cpu load (sys) on my Alix, but did nothing to improve maximum throughput. This comes from the fact, that operations need to go from the system to card and back. It can be seen from increasing interrupt load.
Maybe my setup was too simple (1 OpenVPN tunnel) and performance benefits can be seen, when using the card with multiple clients, but I would just stick some more oomph into the box :-)