Here're the 2010 attendance numbers; unfortunately, capacity isn't listed:
http://www.ncaa.org/wps/wcm/connect/public/ncaa/pdfs/2011/2010 national college football attendance
I would think that true sold-out seasons are rare, but getting 90% of capacity for the season is pretty standard in the big five conferences. Obviously there are teams that don't draw as well (private schools, teams in notoriously unenthusiastic sports regions, programs that have been bad for decades).
1998 was weird. 49,800 isn't a ton of seats to sell; while 47,000 is nice, it's crazy that Tennessee, Rutgers, Cincinnati, Virginia Tech, Boston College, and Miami couldn't achieve 100% capacity. Four big programs and two lousy teams against whom our entertaining offense scored something like 130 points.
That era saw good attendance with few sell-outs; since that time, demographics in Central New York have changed in such a way that we'll possibly never draw routine sell-outs again. I don't think that's a reason to decrease capacity, but it's probably best not to get our hopes up for packed houses. Casual folks have tuned out; the number of Central New Yorkers who care about football has dropped significantly.