What I would do...
1. Focus on long tail (make a list of post titles based on keyword tool results and competitors' blogs and then write decent 300 word articles).
2. Link build (don't spend any money, link to your other sites and find decent blogs to comment on and a few reasonably good directories).
3. Carry on doing the above two things for 3-4 months, plus incorporate webmaster tools and analytics results to see what terms people are using to search.
4. By this time you should be earning money on long tail results, so use the keyword tool again and your existing content to work up the pyramid of shorter phrases.
5. Get advertisers on board at a cheap monthly rate to guarantee income (links based and 'extra results in the search engines' based advertising rather than traffic advertising).
6. If it's not making as much money as you want, then sell, not forgetting to emphasise the established site/original content/tons of backlinks/regular advertisers/PR whatever.
I'm working through this on a site I've been developing since May, it's only making £XX with Adsense at the moment but Google is pretty keen on it and my stats are improving all the time. Depends how much work you've got on though I guess.