I am not a tax lawyer, but I am fairly certain that you MAY NOT file your taxes as head of household. That designation is only for people who are unmarried. You must file either Married Filing Jointly or Married Filing Separately.
Next, if you have been working with authorization (you have an EAD), then you can count BOTH incomes. You will need to file an I-864A, a contract between you and your wife, in which you accept joint liability with her for her contract with the government memorialized on her I-864.
It's a bit crazy but this is what it means. Her I-864 promises to reimburse the government if you use means tested government benefits. Your I-864A promises to share with her in the payment of that reimbursement. Your income only counts though if you work with authorization.
If you now have an EAD but you did not in 2012, bring pay stubs and employment letters showing that you now have a steady, pemanent job.
If you are cutting it very close, find a joint sponsor.
Finally, as I read your question, it seems like you may not yet have filed your I-130 and I-485. Go ahead and file those with your wife's I-864. If her income is not sufficient, you can file new I-864 and I-864 from a joint sponsor when you go for your interview. Get yourself a lawyer to help prepare for that.