Some savvy (and cynical) readers have wondered whether HBO, flush with the success of the "Sex and the City" big-screen movie, would rush into deals where maybe "The Sopranos" or "The Wire" or other TV series would be turned into films.

The thinking is certainly logical, but there are few instances where it makes sense for a creative, long-running TV series to leap to the big screen. Fans can dream it, but what's the real gain?

As to "The Sopranos," which lots of devoted fans are sure will "wrap up" on the big screen, the odds are actually the longest. Creator David Chase will definitely step up to writing and directing films but he has no desire to milk his mob series and become a one-hit wonder, forever identified with a TV series and nothing else. He's not going to squander the chance he earned with "The Sopranos" to take the easy route. He can make any movie he wants now. If one or two efforts fail, maybe then he'd consider taking another look at the Soprano family, but don't bet on it.

David Simon, who created "The Wire," is much more likely to come up with another dense, wonderful television series. Simon proved that he likes to tell a long story - the great gift of serialized television - and his efforts will no doubt be focused there. Besides, any "Wire" movie would have to be a prequel (given how the series wrapped up) and there's just not enough of an audience to finance that.

Certain TV series - most of them sitcoms like "The Brady Bunch" - transfer easily to the big screen. "Get Smart" is the most recent upcoming example, and animated staples like "The Simpsons" and "South Park" have done it. Since Hollywood is perpetually out of original ideas, it's easy to see why it dips into nostalgic, mostly empty action series - "Charlie's Angels," "Dragnet," "Starsky & Hutch" - and turns them into star vehicles that generate easy buzz but rarely edify audiences. "Mission: Impossible" is an exception as a franchise, while "The Untouchables" and "The Fugitive" were one-off successes that reinvigorated classic premises.

The list of crossovers seems endless. Some were comic books and TV shows before becoming movie franchises - like "Batman." For every "Transformers" or "Speed Racer" beefed up for adults, there are other kids TV shows where the producers knew that a movie version - opening in the summer - would almost guarantee a profit ("Rugrats," "SpongeBob SquarePants," etc.).

And while the pipeline from small to big screen no doubt will continue through the years, what's missing from all of these crossovers is twofold: A sense of continuity in the TV story line (a la "Sex and the City") and, most important, a furthering of the quality as told on television.

Meaning that if you take out the box-office appeal of familiarity and focus solely on whether it makes sense for a great TV series to become a film, you're left with far fewer good options.

You can argue that the "Star Trek" franchise was beefed up and made more relevant by the film versions, but that's almost too easy - the big screen is always kinder to science fiction and anything with special effects or hellacious action.

What about "The X-Files"? The original film was highly anticipated because, near the end of the TV run, the series had bogged down in murky mythology and there was a sense that a movie would tie up many of the loose ends that the television finale failed to.

It didn't - not exactly. And maybe that's one reason another film is coming. But at least "The X-Files" had a relevance to it.

As for purpose, the Joss Whedon film "Serenity" pleased its legions of fans because it continued the shortened TV series "Firefly." That's one film where rabid fan loyalty turned a genuine cult series with limited audience into a rare big-screen adaptation.

"Serenity" had a greater goal than "Twin Peaks: Fire Walk With Me," which seemed like a desperate studio giving money to a visionary (David Lynch) in hopes that he could reignite the buzz that surrounded the TV series (at least Season 1). Of course, that didn't work out so well.

Fans of "24" had hoped that the big-screen version would be out soon but the concept has been put on hold (the TV series itself was held back because of the writers strike and won't return until the fall). Though "24" makes sense as an action movie (with a built-in fan base), a two-hour movie ruins the 24 episodes as 24 hours conceit of the series and would negate the show's real heartbeat - the ticking clock that winds down as the action builds.

"Sex and the City" qualifies on the two elements that make for a perfect crossover. It continues the stories of Carrie, Miranda, Samantha and Charlotte - particularly that of Carrie, whose pursuit of marriage to put a bow on her long pursuit of love goes a little sideways in the film. The movie advances the story that was left off when the series ended. It neatly posits that the lives of these four wome have changed in ways that will be interesting to the audience. But in rushing to do that, "Sex and the City" also overdoses on itself and dilutes the charm of 30-minute episodes on television.

That should deter even the dreamiest notions of turning cool TV series - "Mad Men," "EZ Streets," "Deadwood," etc. - into big-screen movies.

Even if it makes sense, the end result might not make it better. *