welcome aboard Stavros
Well first off, check the competiton. You've got the Federal Govt Job Board -
USAJOBS - The Federal Government's Official Jobs Site
To be honest, it looks a bit 1990's and dated to me.
I think a better version is the jobserv board which has a much fresher look and is more "web 2" -
Search for jobs in the USA. Online job search
But to start a new one from scratch is a going to be hard work. Personally, I'd concentrate on a single state, maybe calafornia, and then scale it up.
Is this at the ideas stage, or are you at the early planning stage? If you haven't got to the URL yet, think long and hard. Go for a .us extension and try and get the job or job board keyword into the name. Will make your
SEO life easier