Many of the other HP sites speculate about the house Sirius Black was in at Hogwarts. The books never say that he was a Gryffindor.
Reasons he could have been in Slytherin
All of his family were dark wizards
Sirius was a cunning and ambitious man
He liked making mischief-torturing Snape
I have my theory that he was a Gryffindor, but I will explain why after someone else chimes in on this thread.
I believe that the only reason we know James was a Gryffindor is because J.K. told us in an interview. Just something to think about.
I'm one of those thugs that think Sirius was a Slytherin. I also think it's very possible that James, Peter and Remus were Slytherin, but first of all, Sirius... He was born and raised in a pureblood family. And... well, except the fact that blood isn't water, for 11 years his whole family was making him 'proudly pureblood'. It was only in Hogwarts when he met other ideas and views... but he was already *sorted* then.
Hey Maja I think it would be a great twist if they all were in Slytherin. However, I think that because James had his name in the case for the Quiddich cup and it said Gryffindor, then Siruis would be a Gryffindor. The likelihood of them being best friends and in opposing houses is pretty slim. Glad to have you back.
I'm pretty sure they were all Gryffs; I'm pretty sure James was, at least. And if anyone would support rivalry between houses, it'd probably be James. If he was in Gryffindor and Sirius in Slytherin, they'd probably be rivals always trying to outdo each other. I believe they were all in Gryffindor, but that's just me.
"Mr Moony presents his compliments to Professor Snape, and begs him to keep his abnormally large nose out of other people's business." - Remus Lupin (p. 211, PoA)
u have to remember conversation btween sorting hat and harry in dumbledore's office that it said to harry that it's in
your mind that where u want to go? in chamber of secrets
so sirius would have wished gryffider in his mind