Bought a Mac Pro 3,1 (Early 2008) last week. Got it for AUD$600 (US$450) including a 22" 1080p display and a A1048 keyboard and M5769 mouse.
Of course, a "stock Mac Pro" is a rare creature. This one came with 8GB of RAM and two hard drives (1x 640GB and 1x 1TB). It is only the 2.8GHz Quad-Core model. It also came with two video cards, both the stock 2600XT and thus supports up to four displays, though I have only ever used two, and I usually stick with just the single 27" 1080p display.
I got it home, and immediately upgraded from Yosemite to El Capitan. Did some speed tests and found that the 1TB drive was 20% faster than the 640GB, so I did a fresh install of El Capitan on that drive and made it the boot drive. The 640GB has gone into an old PowerMac G5 which I am fixing up, but that is another tale.
Of course, being a tinkerer, I could not resist doing some work on the Mac Pro. I bought an SSD and an adaptor box and did a fresh install of El Capitan 10.11 onto that and copied all my applications over. Boot time went from about 1:10 to 0:32. It is only a fairly cheap Kingston SSD and is peaking at about 240MB/s read and write. The 1TB drive peaks at 120MB/s. Theoretically, the SATA-II Bus should be able to handle 384MB/s.
I also did some hunting around on eBay for a new keyboard and mouse. The A1048 is actually quite a nice keyboard, but I have a very small desk, and it takes up just way to much room. I managed to get the A1314 Bluetooth keyboard, A1296 Magic Mouse (original type with removable batteries) and A1339 Trackpad all for AUD$160 (about US$120) The new versions of these items would cost about AUD$500 from the apple store, so I am quite happy to have the older versions.
So what else will happen to the 2008 Mac Pro? I have a pair of E5742 3.0GHz Quad-Core CPUs waiting to go in, once the heatsinks arrive. I also have another 8GB of RAM (2x4GB modules) which will bring me to a total of 16GB. I am looking at putting a different SSD into it - the old one will go into my mother's old laptop to give it a performance boost. Either a Sandisk or Samsung - depends what I can get for a reasonable price. I will also be looking into a better GPU. The stock 2600XTs do work nicely, but I would like something newer, and preferably DirectX compatible.
Despite being technically obsolete, I find that real-world usage it feels just as current as any new Mac. It runs the same software, it certainly doesn't feel slow or laggy compared to my laptop (MacBook Pro 2011 Core i5 2.4GHz, 16GB RAM, 240GB SSD) And it very nicely works with bluetooth with the keyboard, trackpad and mouse, which are not all that different to the newest models anyway.
***EDIT/UPDATE***
22/07/2016
I originally put a 120GB Kingston SSD in for the boot drive (OSX and Apps) but I changed it to a 500GB Samsung 750EVO. (Got it on sale). The rear speed is slightly higher. Though I think that it will go into my MBP and the MBP's 240GB Sandisk will go into the MBP. I could get an Apricorn PCIe-SATA card, but I somehow doubt that the expenditure will be worth it. I think the RAM upgrade will prove to be a better investment once that has arrived.
