A bunch of my not-happily-employed colleagues (some are unhappy, some are unemployed) are in the enjoyable place of considering job offers. One of them asked, “How do I know it's a good offer?”
You have at least two options I know of, to evaluate salary options: Payscale.com and Salary.com. (If you know of other free sites, let me know and I will post links to them.) You put in your experience, location, salary and see where you are with respect to their databases. You can see where you fall in the range of people and salary. Armed with that information, you can decide if your salary offer is a good one.
Assuming you have a salary you like, consider a start date. Do you want a week between jobs? I don't always recommend that. If you know you want some long weekends in the summer, why take a week in the winter? If you know you want some conference time, take your week later for a conference. You may not need a week now. If you don't, don't take it. Your hiring manager will be thrilled you wanted to start faster.
Even in a down economy, you have options for negotiating salary and start date.