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Since you state that you are self-employed, you probably do not meet the exceptions to the rule for paying estimated tax (which depend on withholding and credits).
It is difficult for me to figure your estimated tax without knowing your total tax situation. However, assuming that your business loss is your only other income (or loss), and assuming that the dividends are "qualified" and the capital gain is all long term, your approximate tax liability on this income (without the business loss and assuming standard deduction and single filing status), would be $3708. If cap. gain is short term and dividends are not all "qualified" the amount will be higher.
These are a lot of assumptions, and if they are mostly valid, I would pay at least 3000 to the federal gov. as est. payment for 2007. You will probably need to file an exception to the penalty form when you file your return assuming that the dividends and cap gain were all received in the last quarter rather than equally over the year.
Best of luck!