One 6800GT would always be better than two 6600GTs in SLI, no question. I'd much rather invest some money in a high range card than spend the same on two identical midrange cards.
SLI isn't being phased out, mind you I'd like it to be more than anything else. nVidia is including bridging capabilities on all cards manufactured now, and with ATI crossfire being developed they sure as hell won't back out of the market now. Intel platforms didn't support SLI platforms until a short while ago, but this was simply because there wasn't an Intel chipset to support it. The Intel nf4 chips are out in full swing now and nVidia have no intention of pulling the plug on that nor do motherboard manfufacturers.