The Arlington County Board yesterday approved slightly scaled-down versions of two major mixed-use development projects totaling almost $300 million that are expected to change the heart of the county's booming Ballston area dramatically into a cluster of skyscrapers.

The board's unanimous approval of the projects came after almost six hours of debate and testimony. Developers said their plans for the high-rise buildings had been made possible only because of a special zoning the board created years ago to encourage the area's transformation into the county's new "downtown."

The projects, board Chairman Mary Margaret Whipple said, "certainly have accomplished what we hoped developers would accomplish when the new zoning was designed . . . . They're very nice projects."

The two complexes approved yesterday encompass three city blocks and would provide key links between the Ballston Metro station, atop which one of the projects would be built, and the $100 million Ballston Common shopping center-office complex under construction at Wilson Boulevard and Glebe Road.

"This is a significant major step forward in a new downtown for Arlington," said County Manager Larry J. Brown in outlining plans for the developments. "They're not without controversy . . . .[But] this is good stuff. It'll provide an exciting, livable urban environment."

The first project, Stafford Place at Ballston, is a $200 million residential and commercial complex that developer Paul V. Cali plans to build on a two-block area bounded by Wilson Boulevard and North Stafford, Ninth and Stuart streets.

It took Cali six years to assemble the 5.6-acre tract owned by 26 persons, including White House chief of staff Donald T. Regan. In its place, Cali plans to build four 13-story office buildings, which would include almost 61,000 square feet of retail space, and two 23-story apartment buildings.

That project is to be connected by second-story covered pedestrian walkways to the Ballston Common shopping center and the second project approved yesterday, the Ballston Metro Center.

The $75 million Ballston Metro Center, to be built on three acres by Giuseppe Cecchi's International Developers Inc., would include a 27-story combined hotel and condominium development and a 13-story office building; there would be 38,000 square feet of retail space.

The Cecchi project, which involved extensive negotiations with the Metro transit authority, is to be built directly over the Ballston Metro Station on a block bounded by Fairfax Drive and North Stuart, Ninth and Stafford streets.

The projects -- each of which calls for open spaces and large glass atriums connecting various buildings -- are the first mixed-use developments to be built under the new special zoning for Ballston's core.

The zoning's most significant feature is its requirement that at least half of all the building space be set aside for residential uses, a caveat the board created to avoid the stark effect of office-dominated areas such as Rosslyn.

Under the zoning's complex sliding-scale formula for determining heights and densities, developers can build taller and bigger buildings in the specially zoned areas than they could elsewhere in Arlington.

Although Cali and Cecchi took advantage of the special zoning provision to include more development on their parcels, the County Board scaled down the density of the complexes, lopping 23,000 square feet from Cecchi's 732,000 square feet and about 61,000 square feet from Cali's 1.5 million square feet.

Both developers said after the meeting that they were disappointed in the board-mandated cuts but did not believe they would substantially affect their plans.

Cali said the cuts would probably require the reduction of one story in each of the four 13-story office buildings.

The board insisted that only a small amount of space could be cut from the retail segment of the complex.

Although high-rise buildings in Arlington are frequently targets of residents' opposition, the two projects generally drew community praise yesterday from residents who emphasized the willingness of the two developers to work with them.

Some residents argued that the developers should not be given extra height or density, contending that the projects' proximity to the subway stop constituted a bonus.

Others said the quality of the complexes justified the extra height and density as outlined in the special zoning category, and they lauded the developers' decision to allow flexible parking arrangements between the office and residential buildings.

In response to several remarks, the board unanimously adopted a resolution promising to set aside land for a park when it reviews plans for future developments in the area. Board member Albert C. Eisenberg got a pledge from Cali that the developer will try to include a supermarket in the retail portion of his project.

Both developers agreed to explore ways to encourage future tenants to use the subway or car pools as a means of reducing area traffic, which is expected to increase markedly.

Cali said he expected that the first phase of the project, involving construction of three of the office buildings, can begin this year; the entire project is expected to take about eight to 10 years.

Cecchi said he expects to begin construction in August, after the Metro Orange Line that currently ends at the Ballston Station opens to Vienna, freeing space in Ballston.

His project is expected to take 30 months to build CAPTION: Picture 1, Larry J. Brown; Picture 2, Detail of the Stafford Place development, a $200 million residential and commercial project to be built on a 5.6 acre tract on Wilson Boulevard in Ballston.