I agree with Kerry above, am considering one myself with the i5-2400 for less than $100 shipped, as long as I select no drive & just 2GB RAM. I have many unused matched pairs of DDR3 RAM & a few unused 2.5" SSD's, since most of mine are on NVMe now.
The Optiplex 990 was $1,905 when new in 2011-12, and being Sandy bridge means a soldered on IHS, although some says it'll run Ivy Bridge chips, I don't want to go there.
BTW, there's a trick published that can emulate UEFI to allow for the install of a NVMe SSD on a x4 card, the 2nd GPU slot is x4, although PCIe 2.0. Have a couple lower capacity ones from 2-3 years back, the original MyDigitalSSD with Phison E7 controller (too hot for notebooks!). Have the adapters & fan, to include double ball bearing replacements, as the stock fan on the card squeals on boot for a minute.
While I've not tried this yet, just ran across it today, will surely give a shot. Worked on some older MB's than the Optiplex 990. Believe it's easier with Linux, as both Ubuntu 12.04 & Linux Mint 13 64 bit had NVMe support, long before most Windows users had heard of the tech. Plus wasn't included with Windows until Windows 8.1.
https://mrlithium.blogspot.com/2015/12/how-to-boot-nvme-ssd-from-legacy-bios.html
Would run at the same speed as my AMD FX-990 chipset MB's (also PCIe 2.0) & even so, are (still) 3x faster than a 2.5" SSD, provided it works & some has stated it does.
Now for the hard part, finding the Optiplex 990 from the seller with best reviews!
Cat