Though they're often coy when asked about their relationship, Jamie-Lynn Sigler and her boyfriend since last fall, Jerry Ferrara, are getting super-serious. According to an insider, the Entourage actor surprised his lady love with a star-studded birthday soiree at Winston's in West Hollywood — and a diamond ring! In front of party pals like Leonardo DiCaprio and Kevin Connolly, Jerry slipped the glistening jewel on Jamie-Lynn's finger at the May 15 bash. "They are saying it was just a birthday present, but it looks like an engagement ring," the insider tells In Touch. "Engaged or not, they are totally in love!" Five days after the celebration for Jamie-Lynn's 28th year, the actress gushed about her main man over dinner with a girlfriend at Nobu in LA. "She was telling her friend that she wants to marry him," an onlooker says. The former Sopranos star, who split with her husband of two years, A.J. Discala, in 2005, met her beau when she filmed a guest spot on his HBO hit comedy last year. When contacted, Jamie-Lynn's rep had no response.