The National Endowment for the Arts is planning some big doings -- more than 600 events around the country -- in a 20th anniversary arts week commemorating the 1965 birth of the nation's foremost arts agency. NEA Director Frank Hodsoll said Monday he now expects the agency to receive between $163 million and $170 million for fiscal 1986, an increase instead of the heavy budget cuts forecast earlier this year.

First Lady Nancy Reagan has agreed to serve as honorary chairman of the 20th Anniversary Committee, with politically active White House favorite Charlton Heston as committee chairman. Also on the commmittee: singer Marian Anderson, actor Gregory Peck, Kennedy Center Director Roger Stevens, writer Eudora Welty, artist James Wyeth and other national figures prominent in the arts.

National Arts Week, to be officially celebrated Sept. 23 to 29, will be marked with events ranging from an art installation at the Washington Project for the Arts called "Messages From Washington" to the 1985 National Video Festival at the American Film Institute in Los Angeles. Iowa, curiously, will host the most of any single state, about 75 in all. Arts-minded heartlanders can view an exhibition titled "The Other Cubists" at the University of Iowa Museum of Art in Iowa City or hear the Chicago Symphony Orchestra play at the Iowa State Center in Ames.