Hi Vlad,
For what it's worth, I have to agree with the more skeptical members of this thread. Though I concure that the skepticism itself is perhaps unwarrented and counter-productive.
The nature of your questions reflects an lack of 'sensitivity' to market (individual stock) behaviours. Personally, I seldom use limit orders, but when I do, I do so for a reason. And when I use market orders I generally wait until the spread has narrowed to a "reasonable" difference.
As for Ameritrade, I've never used them, but I have used Datek, which I believe uses similar online execution technology. Now I use direct access (Trade Station - you can choose flat fee or by the share) and the difference is night and day. Research and executions aside, T.S. requires fewer clicks because the layout is better. I know this sounds trivial, but when you are waiting to see if your order has been filled and want to watch immediate price action, you don't want to be moving the mouse around for no reason. Also, you will probably get better fills with direct access which should more than pay for any "extra" commissions. If you get a $0.02 better price on a 5000 share order then the cost of Direct Access is no longer an issue - is it?
Finally - START SMALL. Even if you have to take some losses, just think of it as an education. I've noticed my whole physiology changes when I am trading size that is too big for my wallet. But don't take my word for it, it's your money.
best of luck.