Just wanted to add something niiice I found
In Defense of Slytherin: A Different Kind of Courage
An original editorial by Desdemona Black
Regulus, like his brother, is named for a star. Dear Professor Slughorn is kind enough to confirm for us Regulus house (although Sirius comments on his family tree in OotP left us with little doubt), and from the tone of his conversation with Harry seems to rate Regulus ability as on-par with Sirius:
The whole Black family had been in my house, but Sirius ended up in Gryffindor! Shame he was a talented boy. I got his brother Regulus when he came along, but Id have liked the set.
-HBP, U.K. hardcover, pg. 71
The consumate collector at least considers getting Regulus some consolation for not getting Sirius, who we know was one of the brightest pupils of his age. It seems unlikely that Slughorn would mention Regulus (let alone remember his name poor Ron), if Regulus was not someone special too, and particularly unlikely that hed refer to the pair of them as a set if their abilities werent about equal as well.
While Sirius is, appropriately enough, the bright star of Canis Major (big dog), Regulus is the bright star of Leo (the lion). Regulus position within Leo is often referred to as the heart of the lion could the heart of the lion be Slytherin? Perish the thought! Proceeding under the theory that R.A.B. is, in fact, Regulus Black (and there are numerous excellent editorials I might refer you to, in addition to Memersons interview and Emersons later clarification regarding the R.A.B. moment of the interview), this Slytherin almost certainly had the heart of a lion and thats not to say he was incorrectly placed in his house, but rather that the two brothers, Sirius in Gryffindor and Regulus in Slytherin, arent as different as they appear. Here was a Slytherin who saw what had to be done (the defeat of Voldemort) and (most probably) sacrificed himself to see it through. Those cunning folk use any means to achieve their end(SS, U.S. version, pg. 118) emphasis added). Indeed.
Posted by: Sara
So I have no clue who Sara is but you can find that whole text on Mugglenet. If you are interested in it, go ahead and read it. It’s good