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)
AMD Newcastle skt 754 3000+
Elitegroup 755-A2 ATX motherboard
512 DDR RAM
120 gig PATA Hard drive
128 mb AGP 8x ATI Radeon 9250
300 watt 12 volt psu
Antec SLK 1600 case