And they’re off!

At 12:01 a.m. today, the starter’s gun fired and the race to nab the NBA’s top free agents began. Execs from the league’s 30 teams began calling marquee players around the league to woo them to their respective sides, with the hopes of landing the likes of Gilbert Arenas or Corey Maggette.

While neither of the two local teams figures to be in the race for a player of that ilk, it’s an important time for the Knicks and Nets as well. For New York, free agency is the next big test of the Donnie Walsh regime. He’s trying to preserve his cap space for a 2010 run at LeBron James — contingent on finding someone, anyone, to take Zach Randolph. But he also wants to add to the Knicks’ core, and that’s not an easy balance. Word on the street was that he’d only be offering two-year contracts, and that’s likely to turn away most of the players worth having.

For the Nets, their free agent chase is complicated by the fact that several of their own players are also free to sign with another team. Three rotation players from a year ago — Nenad Krstic, DeSagana Diop, and Bostjan Nachbar — may change uniforms this summer. In fact, there’s a decent chance that all three could be gone.

But we may not know their fates for a while. It’s a truism in the free agent game that the biggest dominoes fall first. It’s just common sense: Teams aren’t going to commit their dollars to second-tier players until they know the A-listers are out of reach. Therefore, even the Knicks and Nets will be keeping their eyes on the big kahunas.

The other wrinkle to keep in mind is that nothing can be signed until July 9. Teams will negotiate and players can agree to deals with teams, but those deals are based entirely on the word of each side. Once in a blue moon, most infamously with Carlos Boozer’s 2004 bait-and-switch job that got him out of Cleveland and into a Jazz uniform, the word isn’t kept.

Unquestionably, the biggest fish in this year’s free agent pond is Washington Wizards’ guard Gilbert Arenas, especially now that his teammate, Antawn Jamison, signed a 4-year, $50 million extension at the 11th-hour last night.

With Arenas, however, things may get testy. Arenas is representing himself and apparently wants a six-year deal for the maximum from Washington, but the Wizards would seem to have little incentive to grant such largesse. Arenas was hurt most of last season after struggling to come back from an April 2007 knee injury; given that he’s entirely dependent on his quickness, Wizards general manager and ex-Knick Ernie Grunfeld probably want to see more evidence of health before committing $100 million.

Additionally, it’s not like Arenas has many alternatives. Only three teams project to have enough room under the salary cap to do anything substantial. We won’t know the official cap number until July 9, which is why teams can’t sign deals until then, but based on reasonable estimates, only the Sixers, Grizzlies, and Clippers figure to have dough.

The Grizzlies are unlikely to spend, the Clippers may be as well and will only have about $10 million anyway, and the Sixers only have $12 million and already have an established point guard in Andre Miller. Thus, it seems far-fetched for Arenas to expect $16 million a year from Washington.

Arenas isn’t the only one who will be facing a bear market this summer. Several fourth-year players hit restricted free agency this summer after turning down lucrative extensions worth more than $50 million a year ago, including Philadelphia’s Andre Iguodala, Chicago’s Ben Gordon and Luol Deng, and Charlotte’s Emeka Okafor. None is likely to see as rich an offer this summer as they got nine months ago, which is why nearly everyone questioned their logic at the time. Besides, given their restricted status — which allows their current teams to match any offer within seven days — few other clubs will even bother recruiting them.

The one major free agent who is likely to change teams is the Clippers’ Maggette. He’s bristled under coach Mike Dunleavy, and trade rumors have been a near-weekly occurrence the past two years. Plus he’s lusting to get back to Orlando, where he started his career, and because he is an unrestricted free agent the Clippers have no ability to match an offer. Maggette would have to take a below-market deal to join the Magic unless the Clips did a sign-and-trade, but the threat of such a move may force the Clips into accommodating him.

Among prominent restricted free agents, the most likely to change teams are Gordon — whom the Bulls don’t need now that they’ve drafted Derrick Rose — and one of Atlanta’s Joshes — Smith or Childress. Philadelphia will make a hard push at Smith using all its available cap space; the only question is whether Atlanta’s fractured ownership group gets its act together in time to match the offer. If so, there may not be enough dough left for Childress.

Beyond the stars, the market for second-tier talent is surprisingly lame. As a result, modest talents like Diop and Sacramento’s Beno Udrih both seem likely to command a team’s full midlevel exception. Though the Nets wouldn’t mind keeping Diop, at this price they’ll chuckle quietly and bid him adieu.

Meanwhile, Boston’s James Posey seems to be this year’s likely recipient of the B.J. Armstrong Award, given annually to the player who gets horribly overpaid because he did a halfway decent job for a team that won the championship. Yes, Posey did some valuable things last year. He’s also 31 and averaged seven points a game. Buyer beware.

Where that leaves both local teams is picking through the remaining scraps. But as bad as things look in the midlevel range, there are a number of useful players who can be had cheaper. Golden State’s Mickael Pietrus, Chicago’s Chris Duhon, and Orlando’s Maurice Evans and Keyon Dooling all are likely to come cheaply; in fact, rumors are that Duhon will be one of Walsh’s targets.

Across the river, the Nets must decide what to do with Krstic and Nachbar. Given the additions of Brook Lopez, Ryan Anderson, and Yi Jianlian on draft day, Krstic seems a goner. However, look for the Nets to use Krstic’s restricted status as a means to create a sign-and-trade deal that returns some help on the wings.

Nachbar, meanwhile, seems important now that the Nets are so thin on the wings. If they can retain him cheaply (read: not mess up their 2010 cap space for the Nets’ much-anticipated face-off with the Knicks for King James’s services), he should be a priority.

So although neither local team will figure prominently on the national scene, they still should be active players and are likely to announce a couple deals later this month. And if not, we’ll always have the Great Countdown to keep us entertained. It’s just 23 months and 30 more days until LeBron become a free agent, and the positioning has already begun.

jhollinger@nysun.com