transaction replication using only small portion of network... very slow

i have configured a transaction replication between sql 2k5 databases which are located in two different cities. this replication is very slow to synchronize.. and when i check the network usage, not even half of the bandwidht is being utilized.i want to know how can i setup replication to use maximum bandwith..secondly, sometimes i see number of undistributed commands upto 800000is the normal to have upto this number of undistributed commands in transaction replication.i am using default agent profile with value of 100 for CommitBatchSize