It's heartening to see how good many of America's smaller regional museums are - especially since most must manage with very little money, limited community support, and a public attitude that often leans more toward 19 th-century artistic ideals than 20th-century ones.
The Greenville County Museum of Art here is one of the best of these museums. It first saw the light of day in the late 1930s as a small gallery in the basement of City Hall, grew big enough by 1958 to occupy a mansion, and finally acquired its very own home in 1974.
The latter is a dramatically modern, starkly trapezoidal 80,000-square-foot facility designed by the Greenville architectural firm of Craig & Gaulden. The architects obviously knew what they were doing, for the interior is both handsome and functional, with fascinating angles, intriguing directional shifts, and an overall sense of airiness. Space for both the viewing and the teaching of art (the building also houses the Museum School of Art) has been intelligently utilized with the result that both art and those who study it are well served.
All this would mean little, of course, if the art and the teaching were poor. I can only vouch for the former, but what I saw of it was outstanding, both in the quality and in the significance of the exhibitions within which it is presented.
''Andrew Wyeth: A Trojan Horse Modernist'' is a prime example. It was assembled by Thomas W. Styron, the museum's director, and was placed on view last spring. I have only seen the show's catalog and read Mr. Styron's introduction to it, but it is obvious this was a major and valuable exhibition which should have been seen in New York and other American art centers. This show, whose works ran the gamut from Pollock and Rothko to Smithson and Salle (and which included several Wyeths), was probably the clearest and best argument for Wyeth's ''modernity'' yet presented. It was made doubly effective by the fact that Styron is no hidebound traditionalist but a devoted champion of whatever he feels ''tells the truth,'' no matter how outrageously ''advanced'' or how ''conservative'' it might at first appear.