How to determine how much RAM I can put in my Mac Pro

I think you can put more than 16Gb in your Mac Pro. Apple tends to not put the real number in. Check this page:

http://eshop.macsales.com/shop/memory/Mac-Pro-Memory

RAM prices are ridiculously low these days, better put as much as your budget allows you. You can have 32 Gb for 320$ US, or 16 Gb for 165$.

And since you have 1 Gb modules, you should put them in the slots, like 3 & 7, or 4 & 8, if you intend to use them. i.e.: put your biggest modules in the first slots, and then go down in size the further you go. And RAM modules have to be similar for alternating slot positions, i.e. 1 & 5, 2 & 6, etc...

If I may ask, what is your goal with putting more RAM in your machine?

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.