From the USCIS' website, it says
http://www.uscis.gov/graphics/formsfee/forms/i-864.htm
All sponsors must submit the following documentation with their I-864:
* Proof of current employment or self employment
*
A photocopy or an Internal Revenue Service-issued transcript of a photocopy or an Internal Revenue Service-issued transcript of your complete Federal income tax return for your most recent tax year, or an explanation if it is not submitted. Your W-2s and/or 1099 forms may also be required, see the I-864 instructions for details.
I am not sure which one is the latest / updated info. I did send 3 yrs tax return form + irs-issued transcripts + w2 + the most recent 4 months pay-stub to my wife thou.