What would you change in this high-end 4K gaming PC build?

This would be a very capable 4K system. The 7800X3D is still an excellent gaming processor, and the AM5 platform leaves room for a future CPU upgrade. The RTX 5080 makes sense for high-refresh 4K if you want maximum settings and features, although a Radeon RX 9070 XT could be worth considering if it is substantially cheaper in your region.

For the PSU, choose a reputable 850W or 1000W ATX 3.x model from a good product line such as Corsair or Seasonic. A 1000W unit is not strictly required for this configuration, but it provides more headroom and may run more comfortably during GPU power spikes. The quality and native GPU cable matter more than simply choosing the largest wattage.

I'd put more attention into the motherboard than the original list does. For a premium build, look at boards such as the ASUS TUF Gaming B850-Plus, MSI MAG B850 Tomahawk, or Gigabyte B850 Aorus Elite. They should offer stronger VRM designs, better cooling, and more useful expansion features than an entry-level board.

You don't need to obsess over the exact phase count, though. A well-reviewed board with adequate power delivery, good BIOS support, enough M.2 slots, and the connectivity you need is more important than chasing a specific number. The RX 9070 XT is also a reasonable alternative if lowering GPU cost matters; it gives up some performance and uses slower VRAM, but can offer better value.

The overall direction is solid, but the 360mm AIO is probably unnecessary for a 7800X3D. A good dual-tower air cooler, such as a Thermalright Phantom Spirit or Peerless Assassin, should handle gaming loads well while avoiding pump noise and potential pump failure. If you prefer liquid cooling for appearance, the Arctic Liquid Freezer III is a stronger choice.

I'd also look beyond the ASUS Prime board. A TUF Gaming B850-Plus, Gigabyte B850 Aorus Elite, or MSI MAG B850 Tomahawk would generally offer a stronger feature set, power delivery, and M.2 cooling. For memory, look specifically for a 32GB DDR5-6000 CL30 kit with AMD EXPO support. That is usually a better target for AM5 than simply buying the highest advertised frequency.

For storage, consider a Samsung 990 Pro, WD Black SN850X, or Kingston KC3000, and avoid very cheap QLC or DRAM-less drives for the main boot and game drive. Use a native ATX 3.0 or newer PCIe 5 power supply with a proper 12V-2x6 cable rather than relying on adapters. An 850W quality unit should be enough, while 1000W gives additional headroom for transient spikes and future upgrades.