The Founders Edition Geforce RTX 3080 will fit with no issues (Some larger cards might have issues fitting. The Founders Edition Geforce RTX 3090 should fit.... You might need to remove the Workstation GPU stabilizer bracket because of the card length.  Your 950W PSU will be powerful enough for a single RTX 3090.
I'd like to confirm that the 5820 tower will accept one of the new GeForce RTX 3090 cards once they are released. The 950 W power supply seems sufficient, so I think it would just come down to dimensions. The size of the card is 12.3" (313 mm) x 5.4" (138mm) x 3 slots (from [URL] I don't mind if I can't get the case's side back on, as long as I can run it safely. Failing this, does anyone know if the 5820 will take one of the GeForce RTX 3080 cards that were just released? Maybe someone has already tried to do this. Thanks for your help, K
Will a 5820 tower accept an NVIDIA GeForce RTX 3090?