Mac Pro :: 4GB Memory Shows Up As 2GB?

I have just installed 2 x 2 GB Memory from OWC in my new Mac Pro (early 2008). I have followed the video on OWS�s website in terms of where they should be placed!

Here is what I have tried:

Apple 2 x 1GB in Riser A slots 1 & 2

OWC 2 x 2GB in Riser B slots 1 & 2

OWC 2 x 2GB in Riser A slots 1 & 2

Apple 2 x 1GB in Riser B slots 1 & 2

(It should be 6GB in total)

Both options only show up as 4GB in total, where the OWC is 1GB each. Have tried to reset the PRAM, without any luck.

I then tried to take out the Apple memory, so only the new OWC was in and this then showed up as 3GB. Switched them around ant still only 3GB, but in a different order in the riser! So one on the OWC memory sticks must be faulty.