Really depends on how much the side business makes as a profit.

If you can just increase the withholding from W-2 job to take care of the income tax and self-employment tax you can do it that way, or make the estimated tax payments.

Right now, I'm making about $500-$600/month. I'm trying to figure out how to estimate AGI so I can go ahead and pay quarterly taxes. Would my AGI be my employed + side business income? Or only side business income since I'm filing through an EIN?

AGI would be business plus employment. Remember the self-employed business profit will have self-employment taxes, in addition to income taxes.

Does Turbo Tax have a service where I can calculate those amounts and get my estimated quarterly tax?

Yes,

Go to Federal Taxes
Other Tax Situations
scroll down to Other Tax Forms
then to W-4 and estimates.

Unfortunately I cannot find what you are looking for. Can you post a link?

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