But an important note is that this community is probably nerd heaven... A lot of engineers with to much money that love aircraft and hardware I guess (I know that the flying club I've been a member of for a long time has something like 50% engineers so I guess this group is rather similar).
So we are the "few" in regards of the whole target group that is 500.000 plus "medium high average" users... And looking at the Steam Hardware survey from August 2012 the percent of the "average" gamer that has a GTX 580 GPU is 1.16%, and the number of GTX 680 users is 0.51%. In the sigs here it looks more like 50% 580, 20% 680 and 30% other stuff with high end ATI and a bunch of 560:s.
Check it out yourself. Can't find the SLI/Crossfire usage there unfortunately:
http://store.steampowered.com/hwsurvey