It depend of your budjet, the SAN you buy or have and the distance between the location. for a 4Gbs trueputs it is verry expensive. You can use a Netapp appliance that enable De-duplication for synchrnising you files. also you can use Sunbelt Double-Take to synchronyse data between sites but this is a windows Soft it looks like you aer on linux/unix FS. so this is not a solution.
you can by WAFS (Wan aceleration file systems) I think this is the good meening. WAFS will replicate only deltas between to appliance. Cisco and HP hase verry good WAFS
Their is no cheap solution in this kind of solution. If your SAN is de-dup ready it can hel you but normaly thez are doing deduplication on backups file. but NetApps can do it online with non structured data.