When Prince Charles Williams (that's his moniker) fights Bobby Czyz for the light heavyweight championship of the world on Oct. 29 in Las Vegas, sitting at ringside will be Chris Middendorf, a Washington art dealer who now owns a piece of bonny Prince Charley. It seems Middendorf got into the fight game when a client had to delay buying a major work of art because his money was tied up in Williams.

Middendorf, owner of an Adams-Morgan gallery, settled the deal by offering the client the abstract painting he wanted in exchange for a stake in the boxer. Middendorf wouldn't name the artist whose work he traded because he feels he might not fully understand the artistry of the kind of canvas that fighters are knocked to. It may not be a bad investment since Williams has won 20 of 24 fights, 12 by knockout. Part of the fun, Middendorf says, is being privy to the strategy suggested for Williams by Sugar Ray Leonard's trainer Angelo Dundee. "Some boxers maybe are better investments than painters," says Middendorf, a cautious man.

Marvin Buried in Arlington

Actor Lee Marvin, a decorated World War II Marine Corps veteran, was buried yesterday with military honors at Arlington National Cemetery. The 63-year-old Academy Award winner died of a heart attack Aug. 29 in Tucson, where he lived. He made 21 battle-zone landings during the war, was wounded at Saipan and spent 13 months in the hospital. The private service was attended by more than 40 family members and friends. A Marine Corps honor guard escorted the burial party to the grave site just down the hill from the Tomb of the Unknowns. A seven-man rifle squad fired three volleys; a bugler played taps. It will be several weeks before a marker is erected over the grave.

Bush's Son's Surgery

Hospital Report: Marvin Bush, the 30-year-old son of Vice President George Bush, was listed in fair condition at Georgetown University Hospital yesterday after undergoing "elective abdominal surgery." Bush, who has had previous surgery for colitis, is expected to be released by the end of the week.

Out and About

President Reagan has invited the board of directors of the Corporation for Public Broadcasting to a photo session tomorrow at the White House to mark the 20th anniversary of the Public Broadcasting Act, but the CPB people won't be able to return the hospitality. After the White House ceremony, CPB is hosting a reception across the street at the Renwick Gallery, but neither the president nor any other elected or federal official can be invited, not even those who have been friends of public broadcasting. Under the act, it's against the law for the corporation to entertain them ...

Mitch Snyder, advocate for the homeless, and his companion and associate Carol Fennelly were driving to a debate Tuesday night at George Washington University when they noticed a homeless man topple into the roadway at 14th Street and New York Avenue NW. Snyder jumped from the car to help. Just then, another car occupied by Tipper Gore and Page Crosland, Sen. Albert Gore Jr.'s deputy campaign manager, pulled up. Tipper Gore, founder of a group called Families for the Homeless, knows Snyder and Fennelly and offered to take the man to the hospital. A rescue squad was called instead ...

He had made a nice try of it, but when Peter Holm, actress Joan Collins' most recent ex-husband, failed to show up in court yesterday for a hearing on whether he should receive the $80,000 monthly support he was seeking, the judge dismissed the case. Holm had telephoned the courthouse to say he was ill and in France. Collins' lawyer, Marvin Mitchelson, said he thought Holm had been married over the weekend. Maybe his new bride can afford him ...