What you should be doing is paying quarterly estimates every 3 months; that's what the IRS requires.
You need to sit down with an accountant or CPA and get some help now, before next April 15th and you're wondering where the money went.
3rd quarter, 2007 estimates are due September 15th of this year.
As a rough rule of thumb 10 to 20% should be sufficient.
YMMV