I like the 5501 for the head office, but an Alix box might be a better fit for the branch offices. It's close to the specs of the 5501 and cheaper than the 4801. http://www.pcengines.ch/alix.htm
There are some rough numbers in this post- http://forum.pfsense.org/index.php/topic,14581.0.html
Not scientific, but they give you an idea how the crypto cards help. The Geode boards (5501 and Alix) have a built-in crypto chip. As detailed in the referenced post, the driver got incorporated into 1.2.3 builds, but actually didn't help at all and rendered a hifn card useless to boot. I noted this in a support ticket and suggested making glxsb a module instead of in the kernel. I never heard back, and haven't tested recent snapshots. The glxsb driver may be working in the 7.2 based builds. I would suggest you do some tests before deploying. If you need higher IPSec throughput, you may want to stick with 1.2.2 and a hifn.