Online broker Ameritrade has a reputation for entertaining television ads. So, too, has rival E-Trade. But their stocks have turned in less-than-amusing performance. Just look at how these brokerages performed after reporting results that exceeded even the most optimistic expectations. In the first quarter, Ameritrade netted 306,000 new accounts — nearly equal to the 332,000 new accounts it tallied in all of 1999. Still, the stock price dropped — along with the rest of the Nasdaq