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