I used to mnt bike quite a bit. Front end suspension (forks) are important if you off road. Rear suspension is really only needed if you have a bad back or are gonna do some serious mountain terrain and not just midwest hills. You want to be sure the crossbar is about 1" below your...you know. Disc brakes are nice, but they come at a high price tag. You probably won't need them given the fact that you won't likely be on any steep declines or going 50mph downhill! But they are nice...
Gary Fisher bikes are fairly standard mnt bikes, if you found one of his then you'd be in a good place. Trek is also quite good. Basically stay away from Huffy and anything you'd find in KMart or Walmart. Go to a stand alone bike shop and look around at some names in the general price range and you'd probably get a good idea of what you can afford and what is halfway decent. Mikes Bikes in Palatine is a good shop.
