IMHO memory is not something you want to guess at or skimp on. Apple's prices have always been too high, but there are other reputable sources that can provide solid alternatives.
Crucial has always been my favorite source for RAM (but there are others). Crucial's website has some good tools for discovering exactly the type of RAM you need for your specific model of MAC Pro. Their web site is here:
http://www.crucial.com/index.aspx