Noosh,
My advice would be to NOT get a Ph.D. in the USA to make yourself more competitive for a residency program. A Ph.D. in the US, on average, takes much longer to get than an MD. Average time for a Ph.D. is now 5-8 years of graduate school (compared with only 4 years for an MD). The Ph.D. is grueling too. My Ph.D. thesis was 200 pages long (and that's just the average). At the University of Wisconsin-Madison (where I did my Ph.D.), we had 2 years of classes. On top of it, you have to write manuscripts and publish original research (which is a whole different ballgame than simply studying from a textbook). A Ph.D. with publications and a strong research background will almost certainly make you very competitive for a residency program because of the academic rigor of the process and the novel contributions you would be making to your field. However, unless you are very serious, a Ph.D. is likely not worth 6 years of your time unless you truly want to do research.