Estimating bandwidth required for streaming replication

I have an existing postgresql server that I would like to replicate to another location using streaming replication. How can I best estimate the amount of bandwidth that will be required to keep the databases reasonably synchronized, using the actual workload I have now? I've read through the pg_stat_* list but nothing jumps out at me for an overall transaction rate or amount of data written (except for pg_stat_bgwriter, which has totals since the server was last started). Would the number of buffers (which I believe are 8kB each) written according to the checkpoint log be in the correct ballpark?