With its chips already stacked to the desert sky, MGM Grand here is about to double its bet that Reno will be a gambling spa capable of competing with that other Grand Town, Las Vegas.

The commanding hotel, by far the largest in the saucy little gaming town, will virtually double its room capacity with a 982-room addition to its towering presence. When complete in early 1981, the Reno MGM Grand will have 2,000 rooms -- well, 1,997 to be exact -- but that includes 410 suites.

At stake is a cost that Tom Engelman, director of promotion and public relations for the hotel, said is "indefinite now but about $35 million." Neanwhile, down in Gomorrah, the Las Vegas Mgm Grand will add 750 rooms to make it Nevada's biggest hotel, with 2,900 rooms.

In Reno, however, the Grand sticks out like a very, very sore thumb. And it's effect on this community has been significant.

Engleman said its impact on the local economy is vast and that "even opponents of our expansion would not question that." Hundreds of new jobs, millions upon millions of new dollars and additional services all have come in the wake of the initial MGM construction. Now, with more rooms, the facility will go after convention dollars and individual travelers that the city otherwise could not hope to lure.

"We don't have a strip like Las Vegas," said Engelman. ". . . when we first opened, there were three airlines serving Reno. Now there are 10." Deregulation of airlines had a lot to do with that, to be sure, but there is no doubt that MGM put Reno more indelibly upon the map.

The Reno MGM has the largest casino in America, an absolute sea of slot machines, roulette and card tables that emcompass the space of three football fields. It also has 133,000 feet of meeting space and a grand ballroom that will hold 5,200. In addition, there is a bowling alley with 50 lanes, and an indoor Jai Alai court where patrons can bet.

None of the facilities, however, will be expanded. They were overbuilt with this upcoming addition in mind.

"The hotel has been operating at 95 percent occupancy since it opened in May of 1978," said Joe Esposito, vice president of MGM marketing and sales. "The past summer the occupancy was 99 percent. We have been forced to turn away thousands of potential customers wishing to stay in a first-class hotel."

The hallmark for the MGM in Reno is the stage extravangaza, "Hello, Hollywood, Hello," a nine-act bash with a chorus line of 100, plus a reenactment of the San Francisco fire and quake, a space war, a mockup of a Western Airlines 747 at the Reno Airport, a circus, a Hollywood movie parade and even a waterfall.

One trump the MGM here has over the one in Vegas is the answer to the what-to-do-other-than-gamble syndrome that perplexes visitors to the latter. Reno is half an hour from Virginia City, less than an hour from Carson City and from scenic, year-round resorting at Lake Tahoe, Donner Lake and Pyramid Lake. The hotel offers tours of these and others. There also is a camper city adjacent to the hotel, as well as several peripheral motel and hotel accommodations that feed off the MGM overflow where customers can utilize the Big Momma casino and other entertainment facilities without bedding down at the MGM.