Stay away from the cheap walmart bikes.

I doubt that the stock forks on the Trek/Specialized bikes that you're looking at will be setup for your weight and I don't think that they can be upgraded with heavier duty springs to a suit your weight.

Trek actually does have a 300lb weight limit on their mountain bikes.
http://www.trekbikes.com/us/en/suppo...or_your_bikes/
Pages 15-16: http://cms.trekbikes.com/pdf/owners_...M%20102214.pdf

You need to be looking at the following forums.
http://www.bikeforums.net/clydesdale...-200-lb-91-kg/
http://forums.mtbr.com/clydesdales-tall-riders/