you can get more out of it but save for next major upgrade.
I would say add more RAM @Amazon has $70 4x2GB that I use. So I'd go to 16GB at least for what you do.How To Install and Remove Memory Mac Pro
2x2GB FBDIMM DDR2 667MHz @ $35
Hopefully you have 4 or more internal hard drives, SSD -
and keep those in top shape for system and storage as well as scratch and an ATI 5770 at least.
Nice to have a backup computer as well as system.
Other than security patches always having the latest? if you can do what it was intended to run, no reason to keep upgrading and buying.
My recent experience suggests to sell it while it still has some value.
My early 2008 Mac Pro needed more ram as a result of upgrading to snow leopard 5 months ago. £80. Then the NVIDIA graphics card went (knocks out whole machine). £350 for new card. Have just traded it in for a leased MacBook Pro. They have allowed £460 against it, which is just £20 more than I recently spent on it.
Had I left it another year, it's value would have dropped so low it would not be worth spending any money on it at all.
Sob sob, it gets taken away tomorrow. Strange to think the laptop is faster and more powerful. End of an era!