Should you draft a quarterback early?
In 2007, there were nine quarterbacks with 24+ TDs and 3,200+ yards.
In 2006, there were six quarterbacks with 24+ TDs and 3,200+ yards.
In 2005, there were seven quarterbacks with 24+ TDs and 3,200+ yards.
In 2004, there were eight quarterbacks with 24+ TDs and 3,200+ yards.
In 2003, there were nine quarterbacks with 24+ TDs and 3,200+ yards.
In 2002, there were seven quarterbacks with 24+ TDs and 3,200+ yards.
And keep in mind, in a typical year, the league leader has 30-36 TD passes (31 in '06, 32 in '05, 32 in '03, 28 in '02--look here for each season's leading number: only seven times in history has the leader had more than 36 TD passes). A 50 TD season is an anomoly: I expect the league leader in TD passes in 2008 to have 36 or fewer. So generally, you can expect around seven or eight quarterbacks to throw for between 24 and 32 TD passes, while throwing for at least 3,200 yards.
This means that while quarterbacks are generally the highest scoring fantasy producers, you can find quality quarterbacks without using a pick in the first two rounds.
You have to be able to predict which QBs will be solid in the coming season. Some QB will put up great numbers that nobody expected to (Derek Anderson in '07). Some QB will have high expectations to put up great numbers, and will entirely flop (Kurt Warner in '02, Daunte Culpepper in '05). So you need to be able to guess which QBs will be solid in the coming year, and hope which one might be in for a monster year.
That's why I like to look for QBs that are consistent and reliable season by season. Peyton Manning has never thrown for fewer than 26 TD passes in a season, and he's had 4,000+ yards in 80% of his seasons. In most of his seasons, Matt Hasselbeck averages around 1.5 TD passes and over 200 yards per start. These are the quarterbacks I look to get over the QBs that had a great 2007 season, but don't have much history before that. It's more reliable, less risky.
Look to the Game Logs
But cumulative numbers aren't all either. When I studied QBs this season, I made a list of QBs I would consider drafting (or tolerate as my starter), and noted how many games each had with: 200+ passing yards, 250+ passing yards, 1+ TD pass, and 2+ TD pass. This gives me a sense of both the QB's consistency and his ability to break out for high-scoring games.
I also look particularly closely at yardage, because TDs might fluctuate for a variety of factors, and as I noted above, plenty of QBs will have 24+ TDs. Game logs and yardage are what shies me away from Ben Roethlisberger. In 2007, Roethlisberger threw a bunch of TDs, but he had 200 yards in just 8/15 games, and 250 yards in just 3/15 games. That tells me that unless Roethlisberger gets a bunch of TDs (and 2007 was his first season with more than 18 TD passes, so I'm not ready to count on him for that), he'll be a fantasy football disappointment.
Career game logs (and all stats) are easily accessible at pro-football-reference.com.