One piece of important information that you should have before entering into salary negotiations is the AAUP national salary survey. This gives average figures for assistant / associate / and full professors at schools across the country. Even community colleges are counted.
Note that the figures for schools with large business schools, law schools, or engineering schools are usually inflated as faculty in those divisions tend to make more than those in the humanities or social sciences.
Also, you need to ask whether your school is paying you on a nine-month salary or a twelve-month. AAUP adjusts all of their data to 9-month salaries, so if the school is offering you US$120,000 for a 12 month salary (let me know where!), AAUP will count it as "only" getting US$90,000.
AAUP survey: http://chronicle.com/stats/aaup/