This was exactly what I did -- Masters at a Canadian school, now in a PhD program at a US school. I think having a strong research record with your Masters would help you get admitted because you've shown a proven research track record.
However, having a Masters doesn't automatically help -- if you don't show very good work in a Masters, then I think it could actually hurt you, since the school will know that you are a "mediocre" grad student. So, they might want to take someone straight out of undergrad with more "potential" for being a great researcher over someone with a proven mediocre record. But maybe not -- I'm not an admissions committee member, just considering both pros and cons of applying with a Masters.
In either case, you should expect your masters to count for very little towards completion of your PhD requirements. I basically started all over again last year when I started my US PhD program. One of the schools that accepted me gave me the opportunity to waive a few classes but that school had so many class requirements that even with the waiver, I would still have to take a large amount of classes there! I think in the best case, your PhD school might not require you to retake something that you already took in Canada, but you will have to replace it with a different class.
Also, I found that overall, US programs emphasize classes early on and research later (but not true at all places), so you might find your PhD program very course heavy in the beginning. I found that most US students will take all of their courses in the first 1-2 years, while I know in Canadian programs we tend to take courses throughout the whole degree.