300-500 really doesn't buy you much at all. I know when I was first getting into it I had a big sticker shock. What kind of riding are you looking to do. XC trail riding, urban hucking, DownHill? A decent fork (front shock) alone runs at least 300. The closer you can get to the 800-1000 range the better you will be. Bang for you buck trek, giant, specialized and gary fisher (made by trek now) all have good low end bikes. You should demo a bunch of bikes and just see what you like best. AS far as specs go... I love marzocchi and fox forks, stay away from RST and Suntour forks they make better door stoppers then forks. Shimano LX spec line on your derailers, shifter pods, cranks and brakes will be a good entry level start( XT and XTR even better) Dont get all pumped on disc brakes unless they are quality. The only good Mechanical Disc brakes in my opinion or Avids and they are awsome. Hydros disc brakes are a pain for the new guy so stay away. Start calling around to different shops and ask if they have any last years stuff on sale. Make sure you have some money for a nice helmet and gloves. You may also want some baggy bike shorts (they have nut and ass padding) and a jersey to keep you cool in the summer. I love Giro Helmets. I have an Giro E2 and its my favorite so far. I always try to support the Local Bike shop since they are the core of the industry so check with them and give them the option to price match the online and mega store prices. Even if they only come close i'll buy from them and it has paid off in my benefit in the end anyways with free tool use, cheap hook ups on other stuff and sponsorships.
Here are some good sites to check out:
www.mtbr.com - great reviews on everything bike
www.pricepoint.com - cheap bike stuff
www.morcmtb.org - It is a MN bike forum i'm on. Check to see if there is one for your state.
www.pinkbike.com - just good bike porn
www.speedgoat.com - Have insane deals from time to time
There are a lot of mt bikers in here, most of which are huckers. I'm a singletrack trail rider and urban trials rider.