Armand Hammer, the international capitalist who honestly can say "As I told Lenin . . .," does not live on a small scale. So last night, after the National Symphony Orchestra boomed the "1812 Overture" in his honor, and after 2,500 people had sung "Happy Birthday to Armand," he was so moved he decided to give the NSO $500,000 of his billions.

Just like that. "I don't take time to think," he said at a party in the Kennedy Center's Atrium afterward. "I do it at the spur of the moment." He also, just like that, said he would give $250,000 to Carnegie Hall for restoration. It was in appreciation of Issac Stern, the violinist, who performed with the NSO last night and who is chairman of the board of Carnegie Hall.

Philanthropy aside, Hammer still had time for a little business. In the receiving line, he saw his old friend and lawyer, former Texas governor John Connally. Hammer's eyes shone. "There's a possibility we could make a deal," Hammer said. The two men murmured.

"When are you going?" Connally asked.

"Day after tommorrow," Hammer replied.

"Well, I'm going to be there Friday. Claridge's?"

Hammer nodded, acknowledging his London hotel. "And then to Broadlands," he said, "to meet Prince Charles."

The evening was a celebration of Hammer's 84th birthday, thrown by the symphony he has supported over the years. A thick slice of social Washington turned up. Hammer had invited more than 500 of his friends to sit in the box seats; everybody else paid the regular admission price starting at $17.50. Hammer, who had promised the NSO $250,000 until he announced last night that he was doubling his donation, paid for the reception at intermission and the party afterward.

The crowd was generally Democratic, a group less apt to growl about Hammer's longtime business ties to the Soviets than does the current administration. Hammer, although he is chairman of the President's Cancer Panel, has generally been kept at a healthy distance by a mistrusting White House.

Asked how he's getting along with the Reagan administration, Hammer replied, "Oh, fine." When told that the White House might be uneasy about his Soviet contacts, he said: "I think there's going to be a change. I think Mr. Brezhnev and Mr. Reagan will meet before the year's end. And I think they will agree on the limitation of arms."

It was nonetheless interesting to note that White House communications director David Gergen was among the birthday celebrants. "What do you mean?" Gergen said. "Not everything has to be a political issue." He scanned the crowd for fellow Republicans. "Lookit. Here's Mark Leland from the Treasury Department. And Walter Stoessel from the State Department. And Jim Tossi's here from OMB . . ."

In fact, it was Gergen's wife, Anne, who had dragged him out. "Yes, of course," she said. "He needs his culture."

That included the performance by Stern. "We're old, old friends," Stern said of Hammer. "We've known each other for about four months. I was introduced to him after a concert by Mstislav Rostropovich, and we've had a 20-year conversation over the phone."

Stern, who was released from a hospital in Israel 10 days ago after a gall bladder operation, received a standing ovation. His assessment of his performance? "Like my operation," he said, "I'm delighted on looking back at things that went not too badly."

Food at the concert was, appropriately, birthday cake. Among the scenery: Egyptian Ambassador Ashraf Ghorbal, Democratic statesman Averell Harriman, Sens. Bennett Johnston (D-La.) and Paula Hawkins (R-Fla.), arts patron David Lloyd Kreeger, baritone Robert Merrill and retiring CBS News president Bill Leonard.

Asked what he was doing at the party, Leonard shrugged. "He's around," he said of Hammer. "I'm around. But he's more around than I'm around."

By 11 p.m., the Atrium had become a hot mob of well-dressed bodies. People wiggled around, kissing cheeks and saying how simply marvelous everything was.

"Shirley!" cried Betsy.

"Betsy!" cried Shirley.

"How are you darling?" said Betsy.

Shirley seemed marvelous. They kissed cheeks.

By this time, everybody had forgotten the small but loud band of environmentalists who were protesting outside the Kennedy Center as the concert started. "Armand Hammer Belongs in Jail for Hurting My Friends at Love Canal," said one poster. It was a reference to Hammer's Hooker Chemical Co. that dumped toxic wastes into the upstate New York canal.

For Hammer, the highlight of his evening came when the NSO played "Stars and Stripes Forever" in his honor. Rostropovich grabbed Hammer's hand and pulled him to the conductor's podium, swinging his arm in tune to the music. But Hammer looked a little stiff.

"I was afraid that Rostropovich would give me the baton," he said.