unless you're definitely planning on some kind of failover setup, a single 2cpu box would be both easier to administer and cheaper at the datacenter than two single-cpu boxes, and won't break the bank for hardware costs. (I recently upgraded from a single cpu box to a dual GHz p3 for under $400.)
If you do decide to do some kind of poor man's replication with the logfile, it would be substantially easier to just turn on debug in your nsd config. pg driver will then log all statements. (Unfortunately for your purposes this means queries too but I imagine you could just grep for insert | update...) This would be far easier than tracking down all dml and adding a log statement with each.
