There is a website called http://www.cardiffians.co.uk/ which has lots of photos of buildings in Cardiff some remaining, some gone.
Also there is a Facebook group called (I think) Pictures of Old Cardiff which has lots of photos. Quite often there is a photo of a street scene from the 19th century and the contributors take a photo of the street scene as it is now so you can directly compare and contrast.
Other than that I think the Council has an archive of historical photos although I'm not sure how to access this online. There are probably other contributors on this forum who do.
Take a look at my website: http://cardiffhistory.tumblr.com/
Not been updated in a while, but a few good articles on there about some former buildings pulled down and some others which have been converted/refurbed etc.