If I used an ITIN this year for my wife (married filing jointly), but now have a SSN, what do I use for her AGI on this year's returns?

You will use zero(0) for her AGI if this is the first year of using the social security number.  Yours is what was on your tax return.  Please do not forget to contact the IRS to have the ITIN cancelled.  From the IRS website:

Please notify us that you have obtained a SSN and no longer need the ITIN by visiting a local IRS office or writing a letter explaining that you have now been assigned a SSN and want your tax records combined.  If you write a letter, include your complete name, mailing address, and ITIN along with a copy of your social security card and a copy of the CP 565, Notice of ITIN Assignment, if available.  The IRS will void the ITIN so it cannot be used by anyone in the future and associate all prior tax information filed under the ITIN with the SSN.  Send your letter to: Internal Revenue Service, Austin, TX 73301-0057.