So she needs  I-20, your transcripts (to show you are maintaining good grades), a letter from your department (or your research advisor) and proof of your marriage. In my case they didn't care about marriage certificate (probably they know that people make fake marriage certificates); they looked through entire album (with wedding pictures) and had questions in every other photo such as where did the ceremony take place, who is this in the pic, who in this picture are your parents, or parents-in-law, which one is your and your husband's sibling etc etc. Just to let you know they don't allow DVD inside the interview room, so photo album is the best you have. Since you have assistantship, you don't need to show any property or bank balance (I'm telling this based on my own experience). One more thing I included my TOEFL and GRE score reports with the documents. Hope that helps. No need to worry, be cool, your wife is gonna get visa.

Sparty