First, congratulations on your decision. Cycling can be a wonderful, addictive way to keep in shape, and loose weight.

Aluminum frames can be stiff and harsh for lighter riders, but for folks north of 200 lbs, they can be really nice. Nice aluminum bikes can be found in your price range. Many steel and carbon frames on $1000 bikes can be on the flimsy side. When you're looking at bikes, put the pedal down, and place a little weight on it. You will be able to see quite difference in bottom bracket stiffness for various bikes. If the bottom bracket is not stiff enough, the chain can derail while climbing, or out of the saddle. Make sure they don't put you on too small a frame. The top tube length is important, and watch for shops that will try to sell a smaller frame with a longer seatpost.

Try to find a bike with 105 shifters/deraillers, or if a lessor model, be sure that the bike shop is going to stand behind their performance (and adjustment). Wheels are also very important. You're not looking for the lightest, but a good strong wheelset. More spokes do not necessarily mean a stronger wheel. My strongest wheelset is 20 spoke model. Stick to 23-25 tires, or larger.

The seat is the hardest thing to fit, imho. My experience is that it is a very personal experience, what works well for someone else may or may not fit you. I rarely change seats models, but when I do, I look for a seat that fits my tail bone. The tail bone is actually what supports you. Wide and flat may feel good in the shop, but not on the road. When you're trying out bikes, note which seats you like and dislike.

Be sure and ride whatever you're considering. If something is not working correctly, take it back and make them fix it. If they're not willing to fix a problem to sell a bike, lotsa luck after you buy it. Try to show up at the shop on a weekday morning when they're less busy. Look for last year's model in your size.

If you're looking at used, I'd stay with a bike less than 5 years old. Wheelsets have come quite a way in the last five years. Find out what a shop would charge you to go over the bike. It should be $100 or less.

Have the bike shop show you how to check your spokes, and then buy a spoke wrench. Spoke tension is extemely important to a good wheelset. Ask them to set your position on the bike (not a full fit yet). Measure (to the 16th of an inch) from the tip of seat to stem, middle of bottom bracket to top of seat, and finally top of top tube to top of the tip of the seat. As you ride, you may want to adjust your position. Do so slowly, and in very small increments. Note your changes. This will help with soreness, etc. as you get your body used to cycling.

You've picked a great time to get started, during the TdF. Enjoy your shopping, and we'll see you on the road.