Compared to Jennifer Lopez's July 24 birthday bash—where she arrived in a new Rolls-Royce sporting a new diamond ring—Ben Affleck's 31st-birthday party on Aug. 16, at the hip Hollywood club Concorde, was a modest affair, at least by Bennifer standards. A curious mix of celebs—from David Spade to George Hamilton to Britney Spears—dropped by. So did a small throng of gate-crashers who had downloaded the invitation (and RSVP number) off the Internet. Firmly grasping fiancée Lopez's hand, Affleck was seen wading through the crowd with a frown. But later he was all smiles when embattled singer R. Kelly turned up for a 20-minute serenade. "His mouth dropped open, it was such a surprise," says a friend of the couple's. At one point Lopez playfully boogied with Playboy Playmate Teri Harrison, while Affleck worked the room. The party, says a guest, "was really casual." But as she and others departed in the early morning hours, there was no doubt of the birthday boy's star power. Standing at the valet stand one reveler could be heard wondering aloud how much the multicolored centerpieces would fetch on eBay.