This is probably not the best route due to a few issues.
#1-current Apple OS won't work with older versions of Pro Tools
#2-Native card and HD IO REQUIRE HD software
#3-Your current version 8 software, if it is indeed HD, I believe it will need to be at least HD 8.5 to work with the HD/Native card.
If you wish to stay with the older Mac Pro, it might be better to search for an HD3 package(core card plus 2x Accell cards) and a used licence for HD(8 would be fine, but 10 would be better if the OS allows). The HD IO should also be fine, but used 192 IO's sound fine and are pretty cheap these days.
Keep up the research and pay close attention to which versions of PT will work with your version of OSX