Joining you crazy bunch of guys; I recently ordered a Mac Pro 4.1, 8-core 2.23Ghz system for $430. I see some of you guys have gotten some crazy deals on these things, but I'm fairly happy with the cost. I don't have the system yet, in transit now, but I plan on flashing to 5.1, upgrading to Mojave, doing an NVMe boot drive and adding a usb3 card. Thinking of an Nvidia GT 640 as the cheapest option to boot Mojave. I think the system has 16GB or ram which should be fine for me for now unless there is a good source for cheap ram for these things. I'll probably keep the existing CPU's for a bit, but it seems fairly inexpensive to go with the x5670's or x5675's so very tempting.
Any other recommendations for upgrades/mods?
Well, you haven't told us what you want to do with yours, that would help.
CPUs: I went from 8 cores @ 2.24 to 12 cores @2.93. I do a lot of 3D rendering and the difference was very, very noticeable.
Ram: I would recommend more ram. My philosophy is:
Are all of your slots filled with 16Gb sticks? If no, then you don't have enough ram.
I pick up cheap 16Gb ram sticks off ebay. I paid about $30 per stick.
Video card: You can get a GT 740 w/4Gb of Vram off ebay for around $45. I am more of a Team Red guy, so I have a RX560 I picked up for $100.
I added a Blu-Ray player for $50 and another $50 for the software so I can watch my Blu-Rays whenever I choose.
USB Card: I added one to my system (see sig).
E-Sata card: My external backups are on a OWX Qx2 Mercury drive. I added an E-Sata card so I could back up at a reasonable pace.
Boot Drive: I am still running a Apricorn duo I got a number of years ago. It has 2 250 OWC SSDs in a Raid-0. I have looked at NVMe, but I haven't pulled the trigger just yet.
Future plans:
1. Replace all of my 2Tbs with 4Tbs.
2. If Apple screws up the next Mac Pro, I'll start shopping for a Rzyen system. I need horsepower, not fashion.