If I've already nearly got it pegged in this peak operation (sustained is about 30-50%...so far), and the theoretical limit of the 100 Mb/s fd link is ~12,200 kB/s in and out, do I really need this quad 100 card? Will spreading the network load reduce the CPU load?

Spreading the network load will not reduce the cpu load. Given the output of sysstat in your mail, it probably won't help to add bandwidth, however, if you were to experience a different workload, more cached reads for example, you could easily exceed the capacity of a single 100Mb ethernet.