Yes,
- You must file a joint income tax return for the year you make the choice (but you and your spouse can file joint or separate returns in later years).
- Both spouses must claim worldwide income and not use tax treaty benefits for each year they use this benefit.
Yes, if you will be filing joint you can go to your payroll office and file a new W4 as married.
For additional information, refer to the following link from irs.gov:
nonresident alien spouse treated as resident to file joint
