In a significant Fairfax County victory in the regional battle to attract technology firms to the metropolitan area, a thriving California computer company yesterday announced plans to locate a major manufacturing facility in Reston.

The company, Tandem Computers Inc., which is based in Cupertino, Calif., the heart of Silicon Valley, is viewed as one of the nation's fastest-growing computer concerns. In fiscal 1980, the 6-year-old company reported sales of almost $109 million and profits of $10.6 million, up from sales of almost $56 million and profits of $4.9 million. In 1979, the company was rated by Inc. magazine as the fastest-growing small company in the nation.

Tandem is planning a two-phase project on a 38-acre tract in Reston. The first phase will employ about 300 persons in 200,000 square feet of office space by the summer of 1982. The second phase will bring another 300 workers to an additional 180,000 square feet of office space by 1983.

Initially the company will lease office space in Fairfax County for start-up activities. New employes will be trained during the winter and spring before operations formally begin in July 1981.

The success of the company is largely based on its development and marketing of so-called "fail-safe" computer systems that provide automatic back-up mechanisms should the primary mechanism break down.

The company is also well-known for its unconventional working philosophy, which encourages programs such as a six-month sabbatical that all employes are required to take every four years. In addition, the company's California site features flexible hours, and a volleyball court and swimming pool are available throughout the work day.

According to Fairfax County officials, Tandem was among the California computer concerns targeted by Maryland economic development officials as a candidate for locating along I-270 in Montgomery County. The company also considered expanding to the Boston area.

Charles Carter, a staff member with the Fairfax County Economic Development Authority, said that when Tandem officials first came to talk to that agency, Fairfax was their third choice behind Columbia, Md., and Montgomery County. "We turned them around in two weeks," Carter said.

Jean Boyle, the Tandem regional administrator, said the company was impressed by the "quality of life" in the Reston area. "Reston offered us the right variety of housing and had a site ready to go," she said. The company's management also felt that quick access to nearby Dulles Airport was a major plus for Reston.

The facility will also bring the county about $480,000 in real estate and other business taxes, according to John Herrity, chairman of the county's Board of Supervisors.