I'm a business in Canada (QC) that mostly does business with USA based clients over the web. What taxes do I need to apply?

I understand how taxes work within Canada, but when I am dealing with US customers, what taxes apply and do I need to collect taxes to give to the US gov, Candian gov or both? Do I need to register with the US to collect taxes there? How does the income I'm getting from the US (in US dollars) factor in to my Canadian income taxes? According to Canadian laws, I don't need to collect taxes on my services within Canada unless I make over a certain amount in revenue (40K I think). Does income from the US count towards that revenue? I don't want to get in trouble with the CRA or IRS, but everyone I ask gives me a different answer about how this works.