Umk, SLI gives you minimal preformance gains with maximum price gains; it's only for pplz w/ deep pockets. 2x 6600 GT will not preform as well as 1x 6800 GT; and it has compatiablity issues. SLI doesn't preform as well as it should.
Second of all, as much i'm pleased that you're upgrading AMD, I'd like to caution you against upgrade such a recent system. Many of the best upgrades are yet to come; upgrading such a recent system that's still worth a lot is such a shame... perhaps you'd like to upgrade later?...
One the plus side, if you upgrade now, you can take advantage of your old parts being fairly recent (i.e the 6600 GT and the 1024 RAM)