speaking of which

“Speaking of which” is used to add something that is related to what has just been said. “By the way” is used to add something that is only somewhat related to what has just been said. “Anyway” is used to politely change the topic from what has just been said.
برای مثال، مثلا.

در ضمن:
I won't be home when you get in from school tonight. Speaking of which, don't forget your keys.
امشب که از مدرسه بیای، خونه نخواهم بود. در ضمن، کلیدهای خود را فراموش نکنی.

حالا که حرفش شد

به معنی:
راستی. . .

در باره همین موضوع

حالا که حرفش شد. . .