I have the same one you do (most likely): a 2008 3,1 Mac Pro. In my opinion, these machines are still usable (I still use mine), but they are not worth much or any new investment dollars at this time.
If you must upgrade it, put a little ram in it (don't go crazy, anything more than 4 GB should be fine) and a cheap SSD in it and try to leave it at that (a new graphics card would also be nice, though).
But, on the other hand, think about your budget and options for a newer Mac. Sell it for maybe $150 and put that plus your upgrade budget towards something like a low-end 2017 iMac, if you can stretch that far. That would be dramatically faster and better in just about every way than almost any upgraded 3,1.
(But if you do have a G5 vs. an Intel version, just donate or sell it for the value of its case, etc.)