Wow, so many interesting answers and very few good ones.
Here's the scoop.
Normally, with a margin account, you can only have 3 trades where you enter/exit w/in a five business day period. Four or more trades (plus a % of trade requirement) would make you a pattern day trader where then you'd have to have $25K in your account to pattern day trade.
The 25K requirement is a govt dealie that'll exist at any broker you use. If you click on some of the details/disclosures from a brokerage like TD Ameritrade or OptionsXpress you can see how it works.
That said, since you have a cash account, you don't have to worry about this rule since your allowable capital would be limited to your cash and securities on hand at the beginning of the day.
(In other words, you can spend your account value once) So if you had $10K account balance with $6K in cash and $4K in stocks, you'll be allowed to buy/sell $6K of stocks plus any monies gained from selling any of your $4K in stocks in one day.
So if you bought $4K in stocks and then sold them for $5K, you'd still only be able to buy up to $2K more because you "spent" the $4K on the earlier trade unless you sold some of the stocks you entered the day with.
Hope that helps!