Aldo and Suzie Kelly Salvo call it "the real estate agent's greatest selling tool," because it has enable them to buy a $75,000 five-bedroom brick rambler near Tysons Corner.

David Govoni and his wife, Laurel Bybell, candidly admit that, without it, they wouldn't be on the verge of moving into their dream house on two-and-a-quarter acres in Nokesville, Va. that provides them with the "little bit of country" that they want for their 17-month-old son.

Both couples are benefitting from a relatively new - and increasingly popular - federal housing program under which families can tailor their monthly mortgage payments to their rising income. The program allows them to make lower payments during the first years of their mortgage when their salaries are relatively low.Later on their payments will increase as their incomes grow.

It is known technically as the Section 245 "Graduated Payment Mortgage" program and is a variation of the regular U.S. Department of Housing and Urban Development-Federal Housing Administration (FHA) mortgage insurance program.

The Govonis and Salvos are part of the "sharp increase" in the number of people using the program nationally, HUD officials report. During all of last year, the program's first full year of operation, only 500 applications were received. By contrast, over the first six months of this year, 22,000 applications have already been received, HUD officials said.

Locally, the program is gaining more steam, too, as more and more young couples find themselves priced out of a booming housing market where houses that once cost $35,000 are selling now for up to $70,000. Last month alone, 130 applications for the graduated payment mortgage program were approved here.

Under the program's guidelines, families or individuals can borrow up to $60,000, according to Richard Washington, acting chief of the finance and mortgage credit branch of the area HUD office.

"The intent of the program is to help those who might not otherwise qualify," Washington said. He added that there are "no hard, fast rules" limiting the program to certain types of people with certain incomes, however. Area HUD officials noted that one person who qualified for the program had enough cash to pay for the home outright.

The insured homes also must be owner-occupied, Washington added, and he and other area HUD officials said the department assumes that most families will live in the homes for at least five years. HUD "tries to prevent someone coming in representing himself as the owner-occupant but who really turns out to be an investor or speculator and moves out after the first month and rents it out and buys other properties," said one HUD official.

HUD currently has five graduated payment mortgage plans. Under the most popular of the five plans, for example, for a person who borrowed the maximum $60,000 amount monthly payments the first year would be $407.53, gradually increasing at the rate of 7 1/2 percent each year until the sixth year when the homeowner would be paying $574.91 a month in principal, interest and mortgage insurance premiums, HUD officials said. That sixth year figure would continue for the rest of the 30-year term.

In comparison, under the regular FHA program, if a person borrowed $60,000, the home buyer's monthly payments would be $529.53 from the beginning with no variation, HUD officials said.

The five plans available vary monthly payment increases each year from 2 percent up to 7 1/2 percent, and the increases can be spread out over a period of from five to 10 years.

Harry Piccariello, the real estate agent for Citation Homes who sold the Salvos their Fairfax home, said the program is "great," and estimated that he has sold about $600,000 worth of homes through it since March. His company has sold $500,000 worth of real estate through the program each month recently - about 33 per cent of its volume, he said.

"During the first part of the year, I'd have people who would come in wanting to buy houses they couldn't afford, and when they saw what they could afford, they were shocked," Picariello said. "They just couldn't buy a house. With FHA 245, I'm able to find them a place." Piccariello added that he has had no problem getting applications processed and approved quickly through HUD.

Washington said that most mortgage companies and a few savings and loan associations locally participate in the graduated payment program. "We haven't encountered resistance," he said.

David Govoni recalls that his wife first saw their dream house in Nokesville in late May or early June, and announced to him that they were going to buy it. The $67,500 Cape Cod brick house has four bedrooms and two bathrooms. It also happened to be too expensive, they soon realized.

Govoni, a graduate student-teacher in geology at George Washington University, and his wife, Laurel Bybell, a geologist with the U.S. Geological Survey, said they realized they couldn't get a conventional loan. Bybell earns about $17,520 a year working a 36-hour week, and Govoni estimated that he makes about $450 a month - or about $5,400 a year - with his teaching position and the various odd jobs that he performs and which vary from season to season.

The answer to their problem was provided by their realtor who informed them of the federal program. As a reult, under the mortgage payment program, they will pay $331.34 a month for the first year, a sum well within their financial limits. And it will go up as Govoni's income increases.

"I don't see how people in the lower middle class can afford a good home in this area," Govoni said. "This (program) is the best thing that's happening in housing around here. This program made it possible for us to get the bigger house now that we like."

Thanks to the program, the Salvos, married only last month, are leaving their apartment at 19th Street and Columbia Road NW for a $75,000 home on Olney Road in Fairfax County. Their home is a five-bedroom brick rambler, with a finished basement, swimming pool and two fireplaces.

"We never thought we'd buy a house that large and that finished, with a beautiful landscaped yard." Mrs. Salvo continued. "It's a little dream that happened. This is one of the few ways that young people are going to get into the real estate market. Not too many newlyweds get to start out with a new house - and a mortgage," she added, laughing. "But you've got to jump into it now."

The Salvos said their income would not have qualified them for a conventional loan. Mr. Salvo earns about $13,500 a year as a building contractor. By working as a resident manager of her apartment building and selling Avon beauty products, Mrs. Salvo said she had been earning about $20,000 a year. But now, of course, she will lose the resident manager position and its salary when she moves. Though she hopes to find a good job after the move, the reduced initial monthly payments will be a tremendous help, she said.