Moved this over to the SEO forum.
While I think ralph and ULTiMATE are dead on [search is really about a computer trying to understand if your site is good for a human so be good to humans by being well built, well written and well linked too] in that you'll do just fine on your own, there are times when expertise is good to bring on.
But with all experts, there's a big question to ask first -- what do you want? Or really, what are you willing to pay?
If you want to spend 50 bucks a month your outcome is going to be significantly different than if you have a 200k annual seo budget to play with [although I've seen some great 'cheap' guys and bad 'enterprise' ones so it's not all about the amount you spend].