I don't know how often and/or how intense you intend on riding, but assuming you're looking for at least a front suspension bike that's of serious quality, then I'd recommend, b/c of the cost you're trying to stick to, a Specialized Rockhopper or Stumpjumper.
Both are great entry-level mountain bikes.
The nice thing about Specialized is that they make a great, durable bike that takes a lot of punishment.
Specialized frames are among the best, and the components that come standard on those models are typically Shimano Diores, or possibly a bit better. It'll probably come w/ a Roxx Shox Judy front fork, but the newer versions may have a Fox or low-end Marzochi 3'-5' front suspension fork.
At $400, you'd be wise to look for last year's models still sitting around bike showroom floors, or rather hold off until August-September until the next year's models beginning hitting stores.
Hope that helps.
p.s. Once you graduate school and begin making more $$, I'd suggest you look into a Santa Cruz. I bought a Heckler, last year; I don't have body organs worth as much as this thing...though, given the stuff I ride, it's well worth it.
Enjoy the Trails!!!! - chris