OP, we pay $650/wk for 50 hrs. We also handle taxes, contribute to healthcare costs, cover out of pocket expenses, are considering making a car available for the nanny's use during working hours, and - of course - pay all overtime etc.. when appropriate. So we calculate her gross weekly rate at 40 hours of regular time, and 10 hours of overtime.
She is a citizen, has a clean driving record, tremendous references and lots of experience - including working with twins. We found her through a neighborhood listserv - not an agency. That's why I think we got such a great deal - we're very lucky. I was fully prepared to have to go up to $900/wk but not having to be in that range allows us much greater flexibility to give her more hours, contribute to healthcare, consider a third car so she has one, etc... As the PP said, we found that there was some room for negotiation when we were interviewing nannies, though ours actually requested less than we're paying her. We didn't feel right about paying her so little so we bumped it some, but we still have room to grow for a few years with her (happily).
I also agree w/ the PP in terms of talking about a weekly rate (though you do have to know the hourly straight/overtime breakdown), and her other suggestions.
Don't penny pinch - you have to find someone you like and trust, but if you can do that it a way that would allow you to offer raises and bonuses if he/she works out long-term all the better.
Good luck!