I try and not recommend hardware based on my preferences on the Definitive Mac Pro upgrade guide, but here are my opinions for anyone looking for a cheat sheet on what to buy.
Mac Pro 4.1/5.1 configurations vary quite a bit but there's really only two CPUs to really consider due to pricing. The x5680 is cheap, even with a dual CPU Mac Pro, its roughly $70 to purchase 2 CPUs, making it almost the same price as a single x5690. The x5690 is the best CPU a Mac Pro 4.1/5.1 can house.
- Better: x5680 - 6-Core 12M Cache, 3.33 GHz $35 (used)
- Better:x5690 - 6-Core 12M Cache, 3.46 GHz $60 (used)
GPUs on the Mac Pro are limited to the AMD sphere for 10.14 Mojave and 10.15 Catalina. The RX580s are floating around often for cheap, and they're a good entry-level card. The 560 and 570s are even cheaper, but it is hard to beat the price-to-performance. The Vega 56 is probably the best overall value as its performant and can be flashed to a Vega 64 and lands just shy of the Vega 64 in performance after flashing. The Radeon VII is the king and hard to come to buy. At $700, it's expensive but much mightier in the compute benchmarks than the 5700 XT, making it a much more well-rounded card for video editing and other GPU accelerated non-gaming tasks.
- Good: RX580 $100 (used) or $185 (new)
- Better: Vega 56 $225 $300 (new)
- Best: Radeon VII $600 (used) $699 (New) (Mojave and above only)
Soon the Radeon 5700 XT will be supported for the Mac which occupies a space between the Vega 56 and Radeon VII. The Radeon 5700 XT is great for gaming but it computational scores are low, it performs much lower than the Vega 64 in Blackmagic's Da Vinci Resolve.
There's a lot of storage options for the Mac Pro. The Mac Pro doesn't support bifurcation, so inexpensive dual PCIe cards out a no go. The SanDisk Ultra 3D splits the price and cost intersection nicely besting a lot of the really cheap SSDs like Kingston, although the Samsung 860 is a better SATA drive. Moving the drive to SATA3 doubles the max-transfer speed. The HP EX950 again is another splitting the middle drive between price and performance, in spitting distance of the Samsung Evo 970. NVMe requires a firmware flash for 4,1/5,1s and much more leg work for 3,1s.
- Good: SanDisk Ultra 3D 500 GB $65 + optional SYBA SY-PEX40039 2 Port SATA III PCI-e 2.0 $15
- Better: HPEX950 1 TB - $130 + PCIe card $30
- Best: IO Crest IO-PCE2824 (dual NVMe) $200 and Samsung 970 EVO 1TB SSD 2x $170 or Samsung 970 Pro 512 GB $150 x 2
Not a lot to say, for the 4,1/5,1 buy 1333 Mhz, go for 16 GB DIMMs if you can afford it.
- (Mac Pro 3.1) 800 MHz DDR2, FB-DIMMs
- Better: (Mac Pro 4.1/5.1) 1333 MHz DDR3 $17 per 8 GB DIMM
- Best: (Mac Pro 4.1/5.1) 1333 MHz DDR3 $28.50 per 16 GB DIMM
There's no reason to mess around on the cheaper solutions, Sonnet USB cards are problem-free and do not require external power. The Allego Pro and Allegro 3.1c are the same, featuring two USB 3.1 controllers with 10 Gbps (2.5 GB/s) for 4 ports. The only difference is the interconnect. The regular Allegro has one USB controller for 5 Gbps (1.25 GB/s) total bandwidth.
- Better: Sonnet Allegro USB 3.0 $50
- Best: Sonnet Allegro Pro USB 3.1 Type A $129 or Sonnet Allegro 4-Port USB 3.1c
Wireless is slightly annoying, but there are three options: Use a PCIe card, use a mini-PCIe card, or buy the parts online separately. Honestly, its best to just read the very long upgrade guide.