Forget everything you've heard so far about Werner Herzog's ''Fitzcarraldo,'' which brings the 20th New York Film Festival to a metaphysically rowdy, upbeat end tonight at Lincoln Center. The film will be shown in Avery Fisher Hall at 8:30 and will open its regular commercial engagement tomorrow at the Paris Theater.

''Fitzcarraldo'' may well be a madman's dream, but it's also a fine, quirky, fascinating movie. It's a stunning spectacle, an adventure-comedy not quite like any other, and the most benign movie ever made about 19th-century capitalism running amok.

There is a danger that one's perception of what is actually on the screen might be clouded by all of the publicity surrounding the film's lengthy production in the Peruvian Amazon basin, when Jason Robards had to leave because of illness, Mick Jagger because of other commitments and everyone had to put up with physical hardships that no film is worth.

Mr. Herzog, starting over from scratch with Klaus Kinski in the title role, finally completed the film so successfully that it is difficult to imagine what it might have been otherwise.

Don't be put off by the fact that Mr. Kinski, the most idiosyncratic of German actors, plays an Irishman, one Brian Sweeney Fitzgerald, known as Fitzcarraldo among the Spanish-speaking rubber barons whose company he keeps in turn-of-the-century Iquitos, Peru. Mr. Kinski is as Irish as sauerbraten, but then the rubber barons and other residents of Iquitos and the upper reaches of the Amazon also didn't speak German, as they do in ''Fitzcarraldo.'' ''Fitzcarraldo'' has its own reality.

This Fitzcarraldo is something of a nut and a laughing stock among the other Europeans who have swarmed into the Amazon to make their fortunes in the rubber boom. Wearing a soiled white planter's suit and his hair always standing on end, even after it has been brushed, Fitzcarraldo is an affront to civilized eyes and a jester to the Indians.

His attempt to build a trans-Andean railroad has collapsed after the laying of a few yards of tracks and the importing of a single locomotive to sit on them. When the film opens, he is busy losing whatever shirts he has left in the ice-making business in Iquitos. Financial failure of such consistency, at such a time, isn't easy. It must be pursued with dedication.

Fitzcarraldo's flaw - which is not exactly tragic - is that he has no interest in money for its own sake. He doesn't long for the luxuries of Manaus, the rubber capital of Brazil, whose newly rich burghers send their laundry to Portugal. Fitzcarraldo finds it difficult to pay attention to the petty details of business because he is interested only in what he will do with his profits, which, of course, never materialize.

Fitzcarraldo's goal is to make enough money to build an opera house in Iquitos, more rococo than the one in Manaus, and to import a company of singers headed by Enrico Caruso, whose records he plays on what looks to be RCA's first Victrola. In this wild endeavor he has the patient, loving support of Molly (Claudia Cardinale), the madam of the most popular brothel in Iquitos, who, in her own way, is touched by the madness that inspires Fitzcarraldo.

Though she appears only at the beginning and the end of the film, Molly sets the comic tone of the film that, incorrectly, is likely to be compared to Mr. Herzog's ''Aguirre, the Wrath of God.'' Although both films are set in the same part of the world, and both are scenically splendid and star Mr. Kinski, they are completely different works. The story of Fitzcarraldo and Molly is more reminiscent of ''The African Queen,'' but this analogy shouldn't be pursued. Where ''The African Queen'' is sweet, ''Fitzcarraldo'' is likely to be abrasive. It's not slick.

Supported by Molly's belief in him and by her bank account, Fitzcarraldo sets out to make a fortune by opening up a tract of rubber trees on a spread roughly the size of Belgium. The reason the land comes so cheap is that it is inaccessible, an inconvenience that Fitzcarraldo plans to eliminate by taking a large steamer up one jungle river, hauling it in one piece over a mountain and relaunching it in the river on the other side, which has access to his lands.

This is the film's magnificent centerpiece, in which Fitzcarraldo is joined by his ship's nearly blind European captain (Paul Hittscher), a giant Indian mechanic (Miguel Angel Fuentes), a drunken cook (Huerequeque Enrique Bohorquez), and hundreds of Indians who have their own inscrutable reasons for supporting the project.

These scenes are as extraordinary as Mr. Herzog intended them to be, but the movie contains more. There is the film's introductory sequence, in which Fitzcarraldo and Molly travel in a small open motorboat 1,200 miles down the Amazon from Iquitos to Manaus to hear Caruso - paddling the last mile or so with a single oar. For a few moments, we see the great Caruso (Costante Moret) on the Manaus stage in Verdi's ''Ernani,'' co-starring with a masculine-looking Sarah Bernhardt (Jean-Claude Dreyfuss), who lip-syncs the lyrics being sung by a woman who is very visible in the orchestra pit.

I've no idea whether or not this is based on historical fact but, if not, it's a glorious invention. It's also far more satisfying than the movie's climactic sequence, which, probably because of production problems, appears to be composed of two sets of footage shot some months apart.

Miss Cardinale is not on screen as long as one might wish, but she not only lights up her role, she also lights up Mr. Kinski. Molly's belief in Fitzcarraldo helps to transform Mr. Kinski into a genuinely charming screen presence. This adds a whole new dimension to an actor known primarily for playing megalomaniacal tycoons, international crooks and vampires. He is very fine. So, too, are the supporting actors, including Jose Lewgoy, a Brazilian actor who plays a genially sadistic Iquitos millionaire who supports Fitzcarraldo in order to witness some gaudy new failure.

Mr. Herzog did not set out to make what he calls ''an ethnic film,'' and ''Fitzcarraldo'' is certainly no such thing. Yet the film, beautifully photographed by Thomas Mauch, is an exotic, visual treat. The Indians are a handsome people who remain remote and unexploited, though one of their beliefs - or, at least, a belief attributed to them - is appropriated by Mr. Herzog to make one of those points that, under the circumstances, might well have gone without saying. That is, that ''life is the illusion behind which lies the reality of dreams.''

A small reservation to a big film.

The Cast

FITZCARRALDO, directed by Werner Herzog; screenplay (German with English subtitles) by Mr. Herzog; director of photography, Thomas Mauch; edited by Beate Mainka-Jellinghaus; music by Popol Vuh, Giuseppe Verdi, Vincenzo Bellini and Richard Strauss; produced by Mr. Herzog and Lucki Stipetic; production compa- nies, Werner Herzog Filmproduktion, Pro-Ject Filmproduktion, Filmverlag Der Autoren, West German TV Channel 2, ZDF, and Wildlife Films Peru S.A.; released by New World Pictures. At Avery Fisher Hall, part of the 20th New York Film Festival. Running time: 157 minutes. This film is not rated.

Brian Sweeney Fitzgerald-Fitzcarraldo . . . . . Klaus Kinski
Molly . . . . . Claudia Cardinale
Don Aquilino . . . . . Jose Lewgoy
Cholo . . . . . Miguel Angel Fuentes
Captain ................................Paul Hittscher
Huerequeqe . . . . . Huerequeque Enrique Bohorquez
Stationmaster . . . . . Grande Otelo
Opera Manager . . . . . Peter Berling
Chief of the Campa Indians . . . . . David Perez Espinosa
Black Man at Opera House . . . . . Milton Nascimento
Rubber Baron . . . . . Rui Polanah
Old Missionary . . . . . Salvador Godinez
Young Missionary . . . . . Dieter Milz
Notary . . . . . Bill Rose
Prison Guard . . . . . Leoncio Bueno