My suggestion: Go to a LBS (Local Bicycle Shop) and get fitted. Then ride as many bikes as you can, until you find one that is comfortable. I wouldn't go in with too many ideas about exactly what to get... just set a price limit, and get what fits. A good LBS will get you on something that will suit you.
I'm 6'3" and I bounce between 250 and 300 pounds (currently at 300). I wanted a road bike, and I ended up on a Giant OCR-3. I love it. No noticeable weight loss yet (riding makes you HUNGRY), but I know its coming. I could barely ride for a mile the first week. Smoking and drinking had me dangerously out of shape. But I kept it up, riding to school, to friends' houses, for fun at night... and now I can do 5 miles easy. Its FUN! Such a great feeling of freedom. Clears the mind. Oh, and I'm finding quitting the smoking to be easier than ever before... I can feel the smoke ****ing up my lungs each one I have, and I know that I'm going to pay for that smoke on the next hill I encounter... so now although I do still pick up a cigarette about once a day, I end up taking a couple puffs and putting it down. I can feel it ****ing up my "well conditioned" (highly relative term) lungs, and it sickens me. Now I feel more in touch with my body than I ever did from weight lifting, running, etc. and I just don't want to **** it up. Its nice.
Just get a bike that fits. I recommend a road bike. Biking is the funnest cardio EVER. Its like you're not even working... you're out having fun and you just happen to get a good workout.
Oh, and a tip: Get a new seat. Big guys need a better seat than the ones that come with a first bike. After a serious case of saddle sore, I just ordered a Brooks B17. I recommend that you do so right off the bat... Brooks seam to be best suited for big guys.
Bottom line: just get SOMETHING. Ride it. There's no sense in spending a whole lot on a first bike, because until you get some miles on it, you're not going to know what you really want in a bike anyway. Your second bike is the time to get something that is just right for you... and by then, you probably won't be well over 300 pounds anyway
I recommend a road bike. That feeling of flying on a well paved road is priceless. On a good level gradient, or a slight downhill, pedaling hard... you feel weightless and FREE. Something big guys probably don't feel too often.
My Giant OCR-3 was $650. With a helmet ($50), tire pump, lights, U-lock, and tax it was $800. But there is no need for you to spend that much. Although I wouldn't skimp on the lock (mine was $35), you can get a cheap pump and you can get a cheap helmet at a used sporting goods store. As for the bike, I tried another bike that was $450, and it felt just as good if not better... but it was alot heavier, and I had the money so I went with the more expensive bike.
Another thing to do might be to get fitted for a new bike, take some out for a ride, and figure out approximately what you want. Then look for a comparable used bike (size, type, make), and pick the one that you like best. E-bay could be a big help here.