The first x16 slot will support PCIe 4.0, and the first M.2 slot might as well. You'd need an expensive SSD for the latter to make a difference.
As for SAM (i.e. resizable BAR), I'm afraid I can't answer, because I don't know. The only thing AMD has said is "500 series" chipsets. One could reasonably expect B550A to be included in that, but as well all know, it's actually B450 with PCIe 4.0 enabled. Is SAM supported as is? Does it need a BIOS update? Does it need a BIOS update after AMD officially gives up on the silly idea of restricting SAM to Ryzen 5000 with 500 series chipsets? All questions which don't seem to have any firm answers published where I can find them.
If I were forced to guess, I'd say the Dell machine won't have an option in the BIOS to turn on resizable BAR, so won't work with SAM, at least for now.
Maybe someone who actually owns such a machine with the 6800 XT can provide a definitive answer.
Your PC uses a DELL OEM B550A Motherboard so the best suggestion is to ask DELL SUPPORT and see if it supports SAM Feature with the 5xxx Series CPUs.
According to this Tom's Hardware review of Alienware Aurora Ryzen Edition R10 Gaming Desktop, the Motherboard uses X570 Chipset: https://www.tomshardware.com/reviews/alienware-aurora-r10-ryzen-edition
But this is probably not the Alienware Aurora R10 Model that you have. So it is best to open a DELL SUPPORT CHAT and find out if your OEM motherboard will support SAM with the 5xxx series CPUs.