SLI does scale better but it would be too similar in performance to your old setup to make it a worthwhile upgrade. I would wait for the GTX 570 or HD6950. Both will probably be out in about a month or so.
the gtx570 is going to be a low temp 480 and i think its not going to be cheap
I think it's going to be right around the HD6950 in terms of performance and Nvidia has shown with the GTX 460/70 that they feel compelled to keep their prices competitive with the equivalent AMD cards.