american lab < pakistan lab << no lab
for phd programs (especially with the current economic situtation) there are more applicants than spots, just like medical school. additionally, they are investing a good amount of money in you for stipend. as an international, the first strike against you is that you aren't eligable for many of the federal gvt fellowships that americans are, which means that the school you go to will be footing the entire bill.
all this means, is that schools are going to need a realistic way to fairly judge how good of a candidate you are. schools that get a lot of people from a certain country may have relationships with that country and know how to identify top students. (University of X sends a student to USA School every few years. The professors at U of X know that if they recommend someone who isn't good, people from their school won't get into USA School in the future.) This may or may not be the case with your school... but if it isn't, you may face a difficult time in admissions.
now, if you do very, very well on the gre... that would help. Keep in mind that even many american students take one to two years off after undergrad to either work as a lab tech or in industry before starting their phds, so you won't really be behind.