Since most of your gain will be short-term capital gains your should pay in the third quarter 31% of you gains to Federal estimated income tax. You should also do the same in the fourth quarter.
Short-term capital gains rate is tax at a taxpayer's ordinary income tax rates. For 2008, individuals are subject to an underpayment penalty unless total withholding and estimated payments equal the smaller of:
90% of the tax show on the 2008 return.
100% of the tax shown on the 2007 return (110% if the taxpayer's 2007 adjusted gross income , or AGI, was over $150,000 ($75,000 for MFS)
What was your tax for 2007?
If your husband will pay $7,000 in Federal income tax withholding this year you should pay at least $7,480 in estimated tax payments to be safe and to avoid underpayment tax penalty.