Size and fit is priority #1. Forget at first what name is on the frame. Go to the LBS and ride EVERYTHING. As previous poster stated, your trail and riding conditions can specify same characteristics you want to look for. Find what fits you best. Make the tech fit you for everything you test ride. NOT a sales pitch, but I am a 12+ year MTB mechanic and build frame ups. If you find a frame that fits, feel free to contact me and we can build you a 2000 dollar rig, (or you can), for half the cost if you are dilligent and patient about shopping.
Feel free to contact me with some further question's or suggestions.
As another previous poster mentioned, you can catch a player deal on a DB, owned by Raleigh at Dick's Sporting Good's. They make a 29'er called an overdrive that is set up with a Deore Gruppo for like 500 bucks. An '08 Sortie or Mission if you want hydro's and longer travel can be found there as well for cheap. I am not a huge fan of DB's, as most offerings are quite heavy for their catagories and the fork offerings suck, but you do get alot of bike for the $, and just like BMX, they are easily upgradable.
If it is truly just for fitness and Cardio, light trail or paved trail, consider too a tri-cross bike. I would be happy to give you a few suggestions in that catagory as well.
Happy hunting and let me know if there is anything at all I can do to help.
This post has been edited by bikedave: Apr 13 2009, 03:43 AM