Are you considering entering a fantasy football league that drafts defensive players? My strong recommendation is to stay away from this new phenomenon because it is not nearly as much fun as you think it is. For starters, the best defensive players (especially defensive backs) don't get the most fantasy points. Teams scheme their offenses to attack the weakest part of the opposing defenses, giving the mediocre and low quality players more opportunity to accumulate statistics.
For example, Darelle Revis ranks around 80th among CBs because the ball never gets thrown in his direction, Asamuga ranks around 120th at his position despite being on of the best at his position. Polamalu was the defensive player of the year, so you would think he'd be the most dominant safety, right? Wrong, he ranks about 30th (varying depending on the exact scoring system). ESPN's experts recommend Polamalu as a "spot start against weak opponents". Clay Mathews came in 2nd in defensive player of the year of the year voting, but he's only half the linebacker Jerod Mayo is. In fact when you look at defensive statistics, how is it possible that Mayo was not the run away leader in DPOY voting? Why did he not get a single vote despite accumulating by far the best defensive statistics? Is Mayo 60 pts better than James Harrison? Not a chance.
Offense and defense don't work the same way for individual players. While team defense statistics accurately reflects the effectiveness of an entire unit, individual players is not the same case. On offense, the best players accumulate the best statistics (unless he's stuck sitting on the bench like Turner backing up Tomlinson). Defense is reactionary, and if the QB keeps throwing the ball at the same DB because he's the weakest link, that DB will get more opportunity to accumulate tackles. He might have 8 tackles, but what about the 200 yards and 3 TDs he was burned for? Does your league have a stat for that?
Furthermore, it also pays to draft defensive players from teams with the worst offenses. How can a terrible offense help individual D players accumulate more tackles and stats? Because if a team has a poor offense that can't get first downs, their defense will get more snaps on the field, providing more opportunity for tackles. Again because points and yards against are not IDP categories in fantasy leagues, you will often have players who had 10 tackles and a sack in a game where their defense gave up 500 yards; but you rarely see offensive players racking up big points in games where their offense doesn't gain many yards.
Basically, avoid IDP leagues if you can. They are more tedious than fun, the best players in reality are not the best players statistically, and really is better in theory than in practice.