If you are going to upgrade to a new SLI system. It will either be SLI GTX460s or GTX480s. I don't view a GTX470 SLI setup to be really an option. Not for what he is looking for.
Both have cons though.
The GTX460 SLI upgrade: Won't be that much of a performance boost I don't think from your 285's.
The GTX480 SLI upgrade: Will run HOT. I mean raise the temp in your room by 2-3 degrees hot. And it isn't necessary so really you will just make your computer atmosphere less comfortable.
As I mentioned, I'd wait for Nvidia's next gen Fermi cards. Even more so if you are perfectly happy with your current performance.