The Motobecane 600 HT is a great bike for under $500. You can get one from BikesDirect.com, and they won't charge you shipping, or sales tax for that matter in 49 states.
The Motobecane 600 HT is similar to the 500 HT in many respects, but there are some minor differences. The first thing you will notice is that the 600 HT has RockShox Dart 2 forks, which are very rare to find on a bike for less than $600 at your local bike store.
The other difference is in the shifting - the Motobecane 600 HT comes with an Alivio front derailleur (one step above the Altus) and a Shimano Deore XT rear derailleur. You will also notice a difference in the shifters.
The Motobecane 600 HT also comes with a better wheelset and brakes than the 500 HT. It does cost $110 more, so if it is in your budget, it is a relatively small price to pay for the upgrades you get. Either way, you are going to get a fun bike to ride on the trails, and you won't find it too hard to keep up with the guys on $800 bikes!