You can get a new bike for under $800. Its up to you if you want a new or used one. Like Matt said, you can find a reallt nice used one for that price or a new one for a bit more. Deffinately look to get a 29er, you will love it. All the brands listed above are great, also Trek is a good bike. The Trek is heavy, tho......so I would leave them as a last resort. Gary Fishers are made by Trek now, but still follow the Fisher guidelines so they are a top of list bike. I did a lot of research before buying my bike and this is the way I listed the brands in order.

1 - Specialized
2 - Gary Fisher
3 - Cannondale
4 - Giant

Hope this helps.