Get ready for the feeding frenzy. NFL free agency is coming.

On Tuesday, with all 32 teams under the $123 million salary cap, the checkbooks open up. Some clubs, most notably Green Bay and Pittsburgh, barely will participate, while likely letting top receivers Greg Jennings and Mike Wallace leave for huge paydays elsewhere.

That’s just their style, and building from within surely has paid off: each team has won a Super Bowl in the last five seasons.

Others will eagerly be at players’ and agents’ doorsteps (figuratively and possibly literally) ready to hand out millions of dollars in salaries and bonuses.

It’s not a process former team executive Bill Polian particularly likes.

“You have very, very tough decisions to make. I’ve often said you can’t be right in free agency,” Polian says. “No general manager can be right in free agency.  This system is designed to have you make mistakes (because) the union wants players to get paid, and people are going to make mistakes here.
“So … at least from my perspective, you try to eliminate as many mistakes as possible by taking as few risks as possible.  Some people may see it differently.  And that’s what makes the world go round.”

Already, the biggest potential prize has gone off the marketplace. League champion Baltimore held on to quarterback Joe Flacco with the richest per-season deal in NFL history: $120.6 million over six years, with $52 million guaranteed. Flacco gambled by playing out his rookie contract, and after a Super Bowl MVP performance to cap a sensational postseason, he won.

And perhaps he set a trend.

“A lot of guys are going to look at what Joe Flacco did and say, ‘I’m going to do the same thing,’ ” says former NFL personnel director Pat Kirwan. “And most of those guys will get burned.”

That won’t happen until next year, of course, and this year’s grab bag has its own attractions — albeit no franchise quarterbacks available. Indeed, maybe no starting QBs, unless someone has a lot of faith in Rex Grossman, Jason Campbell or one of the McCown brothers.

While Flacco-type contracts won’t be flung around, 20 or so players could wring impressive salaries out of the bidding wars. Pass rushers Paul Kruger of the Ravens, Cliff Avril of the Lions, Israel Idonije of the Bears and Osi Umenyiora of the Giants figure to be high on general managers’ lists.

Defensive back, particularly safety, has some intriguing possibilities: 2009 Defensive Player of the Year Charles Woodson, cut last month by the Packers; 2004 award winner Ed Reed of the Ravens; and Tampa Bay’s Ronde Barber. All of them are beyond the peak years — Barber turns 38 next month and could retire — making San Francisco All-Pro Dashon Goldson the most enticing prospect in the secondary.

The guys they have to cover are well represented in the free agency pool: New England’s Wes Welker, Tampa Bay tight end Dallas Clark, Jennings and Wallace.

For teams looking for a breakaway threat at running back who also catches passes, Miami’s Reggie Bush is available. If they seek more bulk and power to go with some elusiveness, Steven Jackson has an option in his contract that could free him from the Rams.

Several perennial Pro Bowl players are about to be free: Bears linebacker Brian Urlacher, Dolphins left tackle Jake Long, Raiders punter Shane Lechler and Browns kick returner Joshua Cribbs.

One difference in free agency now is how much each team is required to spend in cash for players. Over the next four-year period, every club must spend an average of 89 percent of the salary cap. So yes, those normally frugal teams such as Cincinnati and Jacksonville could take the cheap route for a couple of years and still get to that minimum.

Several teams have as much as $40 million to spend, with a portion going to their selections in April’s draft. Still, the Browns, Bengals, Colts, Dolphins, Buccaneers and Eagles are in fine shape should they decide to go on a spending spree.

Or they can look for bargains, an approach that has worked often for the Patriots, Giants and 49ers, all of whom have Super Bowl appearances in their recent past.

Among those attractive free agents who won’t max out a team’s cap could be Vikings Pro Bowl fullback Jerome Felton; Jets tight end Dustin Keller; Steelers cornerback Keenan Lewis; 49ers DT Isaac Sopoaga; Chargers LB Shaun Phillips; Titans LB Will Witherspoon; and a group of guards: Buffalo’s Andy Levitre, who also has played tackle and center, New England’s Sebastian Vollmer, and the Jets’ Brandon Moore.

Polian, who built the Bills, Panthers and Colts into Super Bowl teams and now works as an analyst for ESPN and SiriusXM, explains how teams approach the secondary market.

“If we can’t afford player A, let’s see how much different is player B, and how much different is player C,” Polian says. “In the end of the market and in the after-market, again, you’re looking to fill needs almost always with reasonably priced players who are not at the same talent level or production level as the top guys.”

So while there always is a furious splurge by teams in the opening days of free agency, some of the best moves might come weeks later, perhaps after the draft in late April.

“Lots of teams have benefited from waiting,” Kirwan says, “as difficult as it might be to be patient.”