It is the one where Rachel moves out and Chandler moves in! Rachel won't get upset about it! In the end Rachel says 'its the end of an era!' and thats when monica says 'i know'!!
I think this is the one you mean.
I watched the episode where Monica and Chandler got married the other day and when she walks into the hotel room wearing her wedding dress someone says she looks beautiful and she says 'I know!'
In the episode where Phoebe cuts Monicas hair its said. Not in a self satisfied way though. Phoebes halfway through cutting her hair when Monica realises shes cutting it like Dudley Moore and not Demi Moore. Phobes says "oh!, she's got gorgeous hair" Monica replies "I know!!!!!"