I've been running a vB community since last February, about 720 members, 16,000 posts and 1,200 threads. It started by word of mouth, but the content really bought most people in, as one of the forums is a specialist one for a certain set top box.
I wouldn't pay for posts. I'd concentrate on getting the content there and growing the forum slowly.
I see a lot of people start forums with 'Gaming', 'Media', 'Tech' and 'Off Topic' forums and they rarely take off. You get about 20 forums on the board, and they're all very quiet. If I don't think anyone will read or reply, I'm not going to post. I think it's better to start with a few forums, and once you start covering a new topic on the site, or notice a 'Computers' forum is filling up with gaming threads or specialist threads add a subforum or another forum.
I'm working on a project at the moment, and from my experience with my current site I'll be having 4 forums, one for each 'zone'. If in the 'Entertainment' forum there's a huge number of music threads I'd make 'Entertainment' a category, with an Entertainment forum and a Music forum. Each zone goes down in to about 5 areas, the mistake I made at the start is to give everything a forum. :)
For IT stuff I stick to Neowin and Daniweb, but if it was about PHP or Bluetooth I might look at a forum that covers those too. I think if a community has something unique, like rewards or a specialist topic they'll have no problem growing on their own.
Word of mouth all the way! :)
There was a feature in a magazine a few years ago where they did a few simple but very effective things to get 10,000 visitors in a week. It included going to cities with signs, or printing a load of T-Shirts with your site name or URL and walking around giving out leaflets.
You could also do some market research locally. Go out with a load of flyers and a clip board and ask people a few questions about your chosen topic, then give them the flyer. When you have the results publish them on the site or community, then send them to a local newspaper or radio station in the form of a press release?
I'm babbling on...
I just feel that a genuine interest and quality posts will encourage more posts than someone who isn't that interested. The posts may not be of the best quality they could be for the amount of money you're laying out for them.