The Paper Street Audio Company main Mac Pro is still an older maxed out 5.1. It performs great now with these upgrades.
Everything I chose had to be fully Apple supported officially. No hacks, no 3rd party drivers, no workarounds. Everything had to have proven stability and performance. Everything had to offer a great advantage to my more modestly updated Mac Pro. It’s possible to go further with the video card than I did, but the RX 580 exceeds my needs and I couldn’t justify more money on the video card when this machine will be used almost solely for audio production .
Here is a great resource guide to Mac Pro upgrades
http://blog.greggant.com/posts/2018/05/07/definitive-mac-pro-upgrade-guide.html
Processor upgrade information from EveryMac, an excellent reference site with information about every mac model
https://everymac.com/systems/apple/mac_pro/faq/mac-pro-mid-2010-westmere-how-to-upgrade-processors.html
Here is a processor upgrade kit as found on eBay.
https://www.ebay.com/itm/Mac-Pro-2009-4-1-Processor-Upgrade-Kit-to-12-Core-3-46GHz-Xeon-X5690-SLBVX/221616239012?hash=item33995b71a4:g:pUwAAOSwpZRdX226
The absolute best speed increase for drives comes when you put a PCI card that has NVME M.2 SSD drives on it.
https://www.sonnettech.com/product/m2-4x4-pcie-card/overview.html
Here are the popular and well supported NVME drives I chose for my system.
https://www.samsung.com/us/computing/memory-storage/solid-state-drives/ssd-970-evo-plus-nvme-m-2-2-tb-mz-v7s2t0b-am/
This gets the absolute latest iteration of USB 3 on the Mac Pro.
https://www.sonnettech.com/product/allegro-pro-usb31-pcie.html
Upgrading RAM is as easy as can be. You can get up to 128 GB of RAM for a Mac Pro so I did.
https://www.ebay.com/itm/128GB-KIT-8X-16GB-PC3-8500-REGISTERED-APPLE-Mac-Pro-A1289-MC561LL-A-MEMORY-RAM/321961725352?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
This is one of many kits that supplies the exact, Apple supported, fast wifi and newest bluetooth standards allowing you to do much faster downloads and use features like handoff on the mac.
https://www.ebay.com/itm/Genuine-Apple-WiFi-802-11ac-Bluetooth-4-2-Adapter-Upgrade-Kit-Mac-Pro-4-1-5-1/274475421781?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
My GPU choice was an RX 580 8GB . MacVidCards are a reputable supplier of video cards which are not just compatible with Mac Pros, but you can get them flashed so that you can see the boot screen and boot up volume selector which is a big deal.
http://www.macvidcards.com
I moved some SSD drives I had into the 3.5 Hard Disk slots in the Mac Pro. The OWC sleds are perfect for fitting the 2.5 SSDs into the 3.5 SATA HD slots and are by far my preference.
https://eshop.macsales.com/item/OWC/MMP35T25/
Ifixit is always a super helpful reference site with build guides, tools, parts.
https://www.ifixit.com/Store?gclid=EAIaIQobChMI9O6u2pfp6wIVStbACh0mkg2dEAAYASAAEgKuq_D_BwE
My favorite YouTube channel dedicated to Macs and with a lot of talk about upgrades and options is the Luke Miani channel.
https://www.youtube.com/watch?v=o3Bcpxk-f4Y
The most upgradable Mac Pros are the ones from 2009-2012, which are the 4.1 and 5.1 versions. The 4.1 versions can have their firmware upgraded to the 5.1 firmware.
https://www.ifixit.com/Guide/How+to+Upgrade+the+Firmware+of+a+2009+Mac+Pro+41/98985
The MacRumors forums have a lot of discussion and information for those doing upgrades and builds.
https://forums.macrumors.com/threads/mac-pro-5-1-gpu-recommendation-for-os-upgrade.2173165/
Often overlooked, the Mac Pro does have a common battery to retain its time and various settings.
https://www.amazon.com/gp/product/B00D8P5T0U/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1
The last officially supported OS for this Mac Pro is Mojave.
https://support.apple.com/en-us/HT210190
Here is another great discussion thread with lots of specifics about the process of updating an older Mac Pro to run Mojave. Some things must be done in a particular order, and a couple of firmware updates are involved along the way. Recommended reading.
https://forums.macrumors.com/threads/mp5-1-what-you-have-to-do-to-upgrade-to-mojave-bootrom-upgrade-instructions-thread.2142418/