Use the google adwords keyword selector tool. Go to
www.adwords.google.com It will show search volume for the different keywords, suggest others and you can type in a $ amount for how much you'd be willing to pay for placement. I always put $30. Then it will tell you what terms you can bid on and how much it will cost. This will give you a better idea of what keywords pay more. Plug in the words that cost the most to bid on as they will pay the most. When you find the highest $ value be sure to check back at the search volume trends... some morons spend way too much for keywords that almost never get searched! Beware... There should be a higher volume and higher $$$. Even better if there is a lower # of competitors bidding on the search term. You're more likely to be found and rank higher once indexed for your search term.
I've been getting excited about 1plus.net because you can put any ppc, affiliate or other program code into you mini sites there. Even banners and custom content. You keep all revenue. They do put some of there own ads up though. I'm not sure what to think about them today... the server is down over there right now. Along will all of my sites!!! Might move if this happens regularly. Good Luck.