I dont know about Sylvia Browne, but she may be capitalizing on abilities she once had.
I believe in psychic ability mostly because of Psychic Detectives and they get too much stuff right (obviously) and the solve the crime...sooooooo.
Also, the U.S govt. declassified documents affirming remote viewing whuch is psychic ability. In fact i also heard Edgar Cayce was rather convincing.
Personally, i have foretold at least one catastrophic event, i cannot go into details but i would see a simulation at a particular spot just as the area of interest came into view. The other time was walking down a street and the same feeling came over me that i had somehow had once before. Meaning i knew what was going to happen, i knew who i would be walking with, what was said, the smells, the feeling etc. This is not unique and i think everyone experiences this at least one time. The sad part is i believe this ability fades over time. Not saying that I am a psychic, not at all but i do believe this aspect of the brain has never been explained, mostly because there is nothing tangible to link it to(that we know if)- usually there is a physical side to what we can understand, something to examine otherwise it may as well not exist...to us. Imagine if everyone was blind and one person could see, if this person related to us what he saw, this amazing world around us would we even believe it? No, we would probably call himor her crazy.