Rsync to offsite location

Hello,

I'm running GSX 3.x on a SUSE 9.3 host.  I'd like to make a cron job to rsync from GSX host-A to GSX host-B housed in another location.

Given the link between site A and site B is a WAN link (~ 4 T-1s) I'd like to find the most efficient way of only shipping the deltas in the files. 

Is this possible?  Is there another way I should do this?  I'd rather do this on the host  and not within the VM's.  (Since the VMs are being used by developers who knows if cron will be running at any given time)

Thanks,

-Terry