I can attest to the fact that 2 word phrases are the most popular. We have over 80,000 phrases in Overture and Google and "dental plans" and "Dental insurance" are more common than say "Dentist" or "Dental". Search volume across the ppcs and organic search engines in quite high for dental plan related searches, but luckily the bids are reasonable (in the .50-2.00 range.