How to determine how much RAM I can put in my Mac Pro
It looks (according to my copy of Mactracker) that your max RAM is: 16 GB for a quad-core or 32 GB for an octo-core.

You're already using six gigs, so if you have a quad-core machine you can put in up to ten more. They don't make five-gig sticks that I know of, so four gig sticks would be your maximum, for a total of 14 GB. If you have an 8-core machine, you can put in up to 26 more gigs. If you can find 12-gig sticks (which I doubt), you can put an additional 24 gigs in, but it is more likely that you can put two 8-gig sticks in for sixteen more gigs. This will bring you to 20 GB or (more likely) 22 GB of total RAM.