I've come to the point of being done with riding a very reliable yet very outdated and uncomfortable bike and now need to find a new bicycle. However, I have no experience purchasing bikes. I've only inherited or been given what I have ridden. So, I NEED SOME ADVICE.

Here's my situation:
I would mainly use the bike to commute and work out with the potential of getting more involved with cycling in the future. Here's what I have found that I like so far:

Touring bikes
Cyclo Cross
Or commuters
...all with the curve down handles.

I don't need a lot of bells and whistles and I am not really all that concerned with it being super light. The only thing I have liked is the gear changing system that is connected the brake handles where you push one in to change gears up and you push them both in to go down(not sure what they are called). I need something that will last for a long time, requires low maintenance, has some potential to get more serious later on, between the $200 to $400 range, is comfortable (preferably able to take bumps well) and fun to ride

Outside of that I have no clue what I am looking for. Any advice would be so helpful. So here's some questions:

Would single speeds work okay for getting a good work out (they are usually less expensive)? However, I do like being able to change speeds with different terrains or weather conditions. What is important in considering in buying a bike. What might be a few good suggestions with what I have mentioned? Should I stay away from Bikesdirect.com? What brands are good? I truly know no difference between Wal-Mart brand Denali and Giant.

Thanks for any help