9800gx2's in sli is quite formidable in raw performance even compared to a GTX 570 and 580. It would be more of an upgrade for improve efficiency and stability and heat management than performance.
I'd say upgrade them only when they become a problem or doesn't perform as you want them to perform.
and one more question, is dx11 that much better than dx10? would it be worth it to upgrade just for that?