I have a RTX3080Ti, does it fit to my precision 5820 tower? Do I need a external power supply for this? I don't mind to keep my cover open as long as I can keep it running without much performance degradation.
Supported it's supported, but you may have to pick the right form factor since some are more bulky
So did you try it? The 5820 is ether 425 or 950w right? Precision-5820-Tower-Spec-Sheet.pdf (dell.com)
If the 950w PSU version then that sounds like more than enough if the requirement is 750w. GeForce RTX 3080 Family of Graphics Cards | NVIDIA
Since you already have the GPU it would only take a minute to open the case and try to see if it will fit.