Atomic commitment protocols are needed to preserve ACID properties of distributed transactional systems. The performance of such protocols is essential to avoid adding too much overhead to transaction processing. Also, many applications require levels of availability, which cannot be met by using only one backup source (disk or process). Combining these two requirements, this paper presents protocols for use with main memory primary-backup systems, where there are multiple backup processes for each primary. These protocols are evaluated by simulation and verified by statistical analysis. The results show that the best choice of protocol is not static, but varies with the transactional load of the system.