The reason why we have hourly backups is to minimize any loss that might happen. I remember one time where some people complained about a few posts being lost even when we had so frequent backups. Backups every three days is way to rare for such a large database IMO. If we take the last three days then we had 2532 posts, 481 topics and 156 new members.
Besides, in relation to server load it doesn't matter how often we do it. There will still be a latency once in a while. We have plenty of space for the backups. We keep a backup for a week so we always have 336 (one for the main site and one for the forums) backups stored.
Anyways, I have a solution already, but I just can't use it. I need an additional server for database replication. Backups will then be run on the slave server which will then take all the load.