About PR firms - just take care when selecting one if you decide to go that route - simply because a lot of them still send out e-mail blasts which are annoying and tend to be dismissed quickly. So I'd hate to see you throw away good money and not get great results. Ask them what how they work and what they can do for you before they do it; you don't want your startup associated with the ratfinks.
Overall I agree with what Mystalic wrote but wanted to add a couple of things - note my disclaimer, I write for ReadWriteWeb.
Also, if you do want to target bloggers - make it easy for them to write about you. For instance - make sure the site covers all the things we'll want to know - the basics - who, what, where, when, why and how - you'd be surprised how many people forget that blogs have made it super important to write stuff up quickly; if we have to spend an hour just working out what your product is about, chances are we won't bother.