Concurrent connections don't matter so much as total throughput. I'm not sure how much the numbers are reduced once you put traffic shaping on, but when sending in the clear an ALIX box like that can pass through about 80-85Mbit.
As far as concurrent connections go, RAM would only matter in terms of state table size, and you can fit ~100,000 states in about 97MBytes of RAM. I'm not sure if traffic shaping adds any more overhead onto that.