Some Sci-Fi themed ratings:

Some of these I have number of viewers, some I just have ratings, and some I have both. Using both I am sure we could calculate the missing numbers for the rest.

Surface 11.2 million viewers (gained ratings in the 2nd half hour)
Medium 12.7 million viewers (series premiere over 16 million)
Supernatural 5.7 million viewers
Threshold 2.6 rating (9 million)
Stargate Atlantis Mid-Season Finale ("Aurora" and "The Lost Boys") averaged a 1.7 household rating
Battlestar Galactica Mid-season finale ("Pegasus") 2.0 rating
Stargate SG-1 Mid-Season Finale 1.8 household rating
Invasion 10.2 rating (led in by Lost)
Lost 13.8 rating
Ghost Whisperer 2.9 rating (11.4 million)
Night Stalker Unknown