There is a thread elsewhere in this Mac Pro forum that you might find exciting.
SATA Express meets the '09 MacPro - Bootable NGFF PCIE SSD
https://forums.macrumors.com/thread...the-09-macpro-bootable-ngff-pcie-ssd.1685821/
Members chime in about all cMPs from 2006-2012.
The basic idea is to attach a fast M.2 flash card to a PCIe adapter and enjoy wicked fast performance compared to SATA SSD.
It's bootable and supports TRIM (10.10.3+ native, and earlier OS X using one of Cindori's apps).
You could choose a smaller capacity (say 128GB or 256GB) and use it for boot + apps. Or you could install up to 1TB and use it for data, scratch, whatever. Treat it like any other drive and configure to your needs.
Some users are installing into multiple PCIe slots and configuring RAID0, but I wouldn't recommend that on a 2008 3,1. Newer cMPs have more slots with suitable PCIe performance, but the 3,1 PCIe slots are
Slot 1 = PCIe 2.0 = 5.0 GT/s x16 double-wide
Slot 2 = PCIe 2.0 = 5.0 GT/s x16
Slot 3 = PCIe 1.1 = 2.5 GT/s x4
Slot 4 = PCIe 1.1 = 2.5 GT/s x4
Assuming Slot 1 is dedicated to GPU, and avoiding Slots 3+4 because of their lower performance, that means you want to install it in Slot 2.
There are several vendors and parts available to implement this and it can quickly get confusing. If you want to keep it simple, focus on one of these two paths:
1. Samsung SM951 + Lycom DT-120 adapter
It is a x4 lane config, and there is a quirk in the way the Samsung controller behaves when it is seated in the x16 lane slot which means you only get half the transfer rate. However, users comment that all other functions run full speed. In a 2009 cMP or later it actually works better in Slot 3 or 4, but in the 2008 Slot 2 is still your best performance.
Example sources
SM951
http://www.amazon.com/dp/B00VELD92U/
DT-120
http://www.amazon.com/dp/B00MYCQP38/
I've just ordered them and look forward to seeing what they do in a 3,1.
2. Apple + Sintech adapter
Users search for Apple flash memory pulled from certain laptops or the nMP and attach it via a Sintech adapter designed to handle the dimensions of Apple's non-standard components.
The OP mentioned at the top of my post provides excellent info about going this route. There are many different Apple part numbers you can look for (typically on eBay).
I believe Sintech has variety of adapters depending on what you are trying to achieve. I noted ST-A2013SA is one of them. However, since I am going down the Samsung+Lycom path I am less familiar with all of the details.
Note: the Lycom adapter DOES NOT work with Apple flash, and the Sintech adapter(s?) do not work with Samsung.
Hope that helps!
[ To everyone else, sorry for drifting this thread away from it's Nvidia focus. I've been helped greatly by similar "sidebar" comments and thought this was a useful moment to return the favor! ]