Because you are married, you cannot be treated as single. You can file a joint return, or you can opt to file as “married filing separately.” However, if spouses live apart and meet certain tests, one may qualify to file as head of household instead of married filing separately. To do this, you must have a qualifying person live with you (or a dependent parent even if not living with you); a spouse is not a qualifying person.
A retirement account to which up to $4,000 (or $5,000 if you are 50 or over) may be contributed for 2007, but deductions for the contribution are restricted if you are covered by a company retirement plan. Earnings accumulate tax free.