My short-handed response would be that you need about 200,000 or more uniques per month before it becomes meaningful to advertisers. There are sites with less traffic that get targeted ads, but if you're talking about doing something other than AdSense, you really need to be at 200k+ to get meaningful traction in my opinion. Meaingful in my mind is getting more than $5,000 p/month in revenue, btw.