I need help with RAM for new Mac Pro?

I am about to receive the new 8-core Mac Pro. I would like to upgrade its RAM to its maximum capability. I would rather avoid doing it through Apple because of their overpriced items. I looked into other retailers but got lost in the overflow of memory available. I want to make sure I get the best deal out there (not necessarily the cheapest but the best bang for the buck) and I want to be certain there will be no compatibility issues. Does anyone have any recommendations? Also, according to the Apple website the computers comes with 8 slots that can hold up to 32GB. However, some websites offers 8GB that supposedly work with this Mac. Am I still limited to 32GB? Will the 8GB RAM work with the 8-core Mac Pro?