Based on my own experience, if you send the IRS 20% of the gain, then come tax filing time you'll be fine. http://www.irs.gov/payments.
If your state also taxes personal income, then you should make an estimated payment to your state tax authority also. Send the state an amount equal to whatever the highest tax rate is for personal income by your state. The highest I'm aware of is around 8.5% in CA.
