If you want to ensure compatibility, you have to go for an Apple card. Thing is even then, Apple won't certify some cards for some Mac Pros.
2007 is the original Mac Pro from 2006. OWC seem to think the current 5770 will work in your machine. Apple UK sells it for £199. Another problem is that most cards only have the one DVI output these days. If your display has mini displayport its not a problem but if not you'll need the pricey and not always reliable mini displayport to dual link DVI adaptor.
Going back to the RAM, how many sticks do you have total? For maximum speed you put the first matched pair in the top riser (nearest the motherboard), the second pair go in the lower riser, third pair go in the top one and the fourth pair goes nearest you in the bottom riser.