Precision 5820, compatibility with Radeon RX5500XT video card?

Here is the video specifications for all graphics cards that were tested and validated by Dell to be used with the Precision 5820  https://www.dell.com/support/manuals/en-us/oth-xlt5820/precision_5820_om_pub/video-specifications?guid=guid-1953ea25-a89a-4d21-a84f-d945ab6bd238&lang=en-us

With the 950w power supply, this system can support x2 PCIe x16 Gen 3 graphics cards.  Maximum 600W total of up to 2x 300W double width, full height, full length graphics cards.  Therefore, the Radeon RX 5500XT should work fine with your upgrade plan.

My concern wouldn't be power requirements: check the new card's height, and how the PCIE cable/connector adds to its height or length.   There's not a ton of space in the 5820, and a tall card will top out. 

I'd probably also consider a faster card than the RX-5500-XT. The UserBenchmark website allows you to compare the speed of any GPUs as benchmarked by its user community. The RX-5500-XT has been tested in thousands of configurations since 2019; unfortunately it's slower than the Quadro P4000 that came in my 5820 Tower (2017). 

The RX family offers many great performers at ~$200, $400, and higher prices. Look carefully at images of the GPU you are considering. Pay attention to where the 6- or 8-pin PCIE power connector is located on the GPU. A topside PCIE connector on your GPU might not allow your 5820 case to close.

HTH, & best--

steveg