I did a search when I bought the ram. Showed how to install it. I bought 2 2 gig sticks to go along with my 2 1 gig sticks that came with my mac pro. The search told me to put the 2x2 gigs in riser a in slot 1 and 2, and put the 1x1 sticks in riser b slots 1 and 2. I did this, Vista says I have 6 gigs. I booted up to OSX and it says I have 4 gigs. Under "system Info" it shows that all of riser B is empty. Any ideas?