semi-daytrading = you can buy and sell the same security in the same trading session up to 4 times in 5 business days. It can be different stocks but any more than 4 buy/sells in 5 business days and you are classified as a 'pattern day trader'. Then you must have $25k in your account (I think a combo of cash and/or margin).
http://www.daytradingworld.com/day_trading_rules.html
So, if you have an online broker that doesn't require funds to be cleared before making another trade (Etrade), you can buy, sell, buy in the same day with all of your funds. Ameritrade looked attractive until the guy laughed at me when I asked him if funds were immediately available and then he told me the SEC is trying to phase out this kind of activity. scottrade is the same way. Clearing takes about 3 days.
edit:
www.level2quotes.com <----

and buy some RBAK today; I want out
edit2: Etrade is $14.95 for market orders, $19.95 for limits and stops - it used to be a 5000 share max then a per share cost but I've put in 10k share orders and it shows me a $19.95 limit order cost + $3 <???> fee. So $22/trade, $44 to get in/out; I don't think I've EVAAR used a market order.