Adding memory to 2006 Mac Pro

I recently purchased a 2 gig kit of 667Mhz FBDIMM RAM modules from OWC. I've been having trouble getting my computer to recognize the full amount of memory installed (should be 5 gigs, or 4 gigs with the original 2x512Mb sticks removed)

I had purchased the same 2 gig kit from OWC before and have had it running to date with no problems.

I've made this diagram to help explain the different combinations I tried. One particular combination was with only the new memory installed, it would only show up as 1 gig - with either stick in A or B slots.

http://i298.photobucket.com/albums/mm275/dcomitini/ram.png

I think I've covered everything, please help.

Thanks