My husband came to the states on a K-3 visa. We are in the process of filling out the AOS papers. I want to know which year taxes papers do we need to use.... I still haven't filled out my tax return for the year 2007.

you are required to send copies of last three years tax papers but as for 2007 u would do it in 2008 april.
so u need to send 2006 ,2005,2004 copies of tax papers

Correct, no one will ask for the 2007 taxes before April 2008.

Yep..deadline for tax year 2007 is April 2008 but you could file early too..

If you are filing for AOS before April, I don't see a need to submit 2007 taxes unless there are issues(say not meeting poverty guidelines etc..etc..) with the previous tax years and you want to include the current year then its beneficial.