Grey's Anatomy star Kate Walsh now has another reason to celebrate: In addition to getting her own spinoff, she's also getting married. "I'm engaged," the actress, 39, told PEOPLE on Tuesday at the Entertainment Weekly upfront party at The Box in New York City. "Alex [Young, a production co-president at 20th Century Fox] asked me to marry him and I said yes. We were in San Francisco last week and I'm very, very, very happy." So what makes her fiancé so special? "Oh, everything," gushed Walsh, flashing her Neil Lane Jubilee-cut diamond. "He's the most amazing man I ever met. We're in the process of planning the wedding." Walsh recently stepped out with Young at an April 28 fund-raiser for Barack Obama in Los Angeles. "He's a hotshot," a friend told PEOPLE at the time. "But Alex also has substance. He's a good guy." Meanwhile, ABC this week formally announced that Walsh will star in her own Grey's Anatomy spinoff, playing Dr. Addison Montgomery-Shepherd in Private Practice. The show will air Wednesdays this fall and costar Taye Diggs and Tim Daly. Walsh told PEOPLE the cast will start shooting the series in July. "It's super exciting," she said. "Grey's is about interns starting their careers and starting their professional lives, and Private Practice is going to deal with people who are successful in their private lives." Her character, Walsh said, needed a change of scenery to improve her personal life. "How much more [could] Addison do at Seattle Grace? Her marriage failed, McSteamy and her did not work out. She tried to hook it up with Karev. She's either going to do some light filing or move on." Her new castmates are thrilled about Walsh's engagement. Merrin Dungey, who plays Addison's best friend, was early to notice Walsh's new bling, she told PEOPLE at the EW party: "I was blinded. I saw when she walked into the room and I said, 'Jesus.' It's very exciting." (When Walsh later passed Dungey on the dance floor, they double high-fived.) And Diggs, who partied on with his costars into the wee hours at the ABC afterparty at Tenjune, characterized his own reaction to the news: "So excited!"