Bonus pay is usually taxed the same as your regular Payroll, so whatever you set up on your W-4.
It's added to your income on your tax return, and taxed at the rate your total income dictates, which may be higher with the income increase. For instance, if you're normally in a 15% tax bracket, the addition of a large amount of income could increase your tax rate to 20%.
It's always a good idea to contribute tax-free to your 401K; however, you may need to set it up with Payroll in advance so you don't have an Excess Contribution, if you're already contributing through payroll deductions. Or you could contribute to a Traditional IRA with your bonus income.
Here's more info on Retirement Savings and Tax Rates, and How Bonuses are Taxed.
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