I'll offer you a different viewpoint on the Mac Pro 2008 and its video editing capabilities. You never said what type of video you are editing. There is a difference between 1080pHD video editing and 4K, 6K and 8K video editing either in Pro Res, DNxHD/HR, h.264 or h.265 10bit format. Basically, the 2008 when upgraded properly "CAN" be used as a video editing platform for Premiere or Final Cut ProX. You will have to use Dosdude's patch to use High Sierra on the 2008. The only limitation with the 2008 Mac Pro is that the system architecture is too slow to support more modern GPU cards like the Nvidia GTX TitanX or the 1080Ti or the AMD Vega64 where they have a higher video ram ceiling -- larger than 4Gb of ram. If you're editing below 4K, 4Gb of GDDR5 ram is more than enough and video editing won't be choppy. But you will be limited with 1080pHD and some basic 4K editing and some video editing freelancers in our area "STILL" use the 2008 to make money. Not everyone is flushed with lots of cash to do temp gigs with video editing these days. If you're editing higher than 4K, then having 8Gb of ram and up (12 to 16Gb of video ram) is better.
If you're mainly editing 4K and up, then skip the 2008 and go right into the 2009 to 2012. 12 cores vs the stock 8 cores is really mainly about CPU encoding speed, because encoding and decoding h.264 is mainly done by those CPUs. Only the rendering and the color grading portion of your video editing are done via the GPU. "Majority of all" video editing software offered today in 2019, are multicore and multi-threaded. The upper limit of GPU that the Mac Pro 2008 can use without causing the CPU bottleneck is the GTX770, which is a Kepler card. But that card is still capable to do some heavy duty video editing "BETTER" than any 2009 Macbook Pro or 2011 Mac Mini, because in video editing, having a capable GPU counts a lot.
So if you tell us what are your plans in regards to the type of videos you are planning to edit, then we can better ascertain your needs. Lastly, I'll link a video of a lady in Japan who made a good video explaining the video editing capabilities of the 2008 Mac Pro and in a part 2 of the episode (2 years later) upgraded her 2008 Mac Pro to 28Gb RAM, a GTX770 and running Mojave and the thing screams in video editing. So just be aware that for modest budget needs, the 2008 is still capable. I personally own the 2009 flashed to Mac Pro 5,1 and the reason was I needed the Radeon RX580 support, higher ram ceiling on the GPU for a few photographic applications that need them that the 2008 won't be able to deliver performance wise. Video editing is certainly nicer with the Mac Pro 5,1 than with the 3,1 2008, but I think that as long as you understand your needs clearly and match a Mac Pro that can address your needs, why not save some some money in the process? Her use of Firewire drive clearly tells us that she's not doing any high end 4K editing and up as the drive is too slow to process 4K RAW and up.
Link to the 2008 Mac Pro review for video editing is below part 1.
Here's part 2