N2
そういえば
Speaking of which, By the way, Come to think of it
そういえば、大家さんに家賃振り込んでくれた？
Speaking of which, did you transfer the rent to the landlord for me?
Explanation
Means 'come to think of it'. Hearing what the other person says, or seeing something nearby, you suddenly remember a related thing or something you forgot, and bring it up: 「明日は田中さんの誕生日だね。」「そういえば、プレゼントをまだ買っていなかった！」 ('Tomorrow is Tanaka's birthday.' 'Come to think of it, I haven't bought a present yet!'). The tone is relaxed and natural, very common in daily talk.
Mixed up with
「そういえば」: the new topic must be linked in your mind to what was just said; the remark woke up a memory.
「ところで」: a topic changer, 'by the way'; it can have no link at all and jump to a fresh subject. 「今日の天気はいいね。ところで、来週の件だけど…」
「それにしても」: mostly a concession (even so, that's too much), with surprise; when it moves on, it is to your own impression (「それにしても、大きくなったね」), not to something you just remembered.