I'd like to see a little more about the composition date and manuscript tradition of those texts before taking them as gospel. And maybe I was missing something, but a quick romp through the life of St. Columba didn't yield much for me. (admittedly, it was a very quick romp! ) And remember, saint's lives are hagiographical, not historical -- the life of St. Columban (with an N) is also the earliest source for the Loch Ness Monster.
I wasn't particularly impressed by the Celtic Twilight site. Lots of wishful thinking, but not much history.
I think this BBC site does a pretty good job of taking one through the early sources, and showing how one might *wish* Arthur was historical, but that none of the early sources actually prove that.
Each page (there are 7) are quite short, but put together, I think they're quite persuasive.