For a big guy like yourself who will be riding primarily offroad, I would look at something a bit higher end than the Diamondback above. $1000 will buy a lot of bike these days. Check out the Kona bikes, their cross country hardtails or the Out of Bounds series. For the price range you are looking at you will get a Marzocchi fork (good for big guys, very reliable) and disc brakes as well as a frame that has a solid reputation for taking abuse. Konas tend to offer a good bike for the money.
http://www.konaworld.com