I too am a foreign undergraduate (Green card holder from the UK). I am currently half way through my 6th year of my immunology Ph.D and feel that I am in a position to offer you some candid advice.
Despite being about to graduate with a doctorate in a US university my undergraduate degree is not recognized by the school I applied to. I have had to retake several undergraduate courses that were not covered by my grad school courses. I am currently wait listed.
While I enjoy my work, the lack of satisfaction (there is no sense of completion) and lack of job security urged me to combine both fields. I would love to do both research and dentistry.
Anyway, with respect to doing a Ph.D. to improve your chances of application I can honestly say do not waste 6 years of your life. Only embark on a Ph.D. if you love research. Beginning this path for any other reason will be miserable for you. The courses aren't too hard to get into but are very hard to finish. The average is 6 years in the US. Consider that your standard of living will not be elevated for a decade while completing your 2 doctorates.
Research is a wonderfully interesting and self governing career, however, it offers no job security whatsoever. If you don't publish you perish. Many do; the field is necessarily competitive.
While you may make a good candidate for a Ph.D. course please realize it is you who is doing them the favor when you join. You will work hard and long for minimal pay. When you finish - dental school aside - you will go on to work long and hard for little more as a 'Post-doc'. Post-docs last 2-3 years and you will be expected to do at least 2 of them before you will be considered for a faculty position or a job in industry.
This is a long path. Enter it for no other reason than for love of research.
A masters degree will likely get you where you want to go in 1/3 of the time. If you don't seriously want to do research then do a MS and get where you want to go. The Ph.D may appear cheaper (I get paid 24K/year) but it will cost you 6 years and compared to a masters it will mean you enter dentistry 4 years later. Each year you delay entry means another year of missed income as a dentist - an income that would earn you far far more than a grad student would get.
Whatever you decide, best of luck.
Paul.
