Start with the graphics card
At 1440p and 4K the graphics card decides your frame rate, so choose it first and size the rest of the build to it. An RTX 5080 is a strong 4K card for most games; the RTX 5090 is for the highest settings at 4K and for creative work that fills its 32 GB of memory.
Then the processor
For games, a gaming-focused processor such as a Ryzen 7 9800X3D is enough, and more cores add little to frame rates. If you also stream, edit or render, more cores start to pay for themselves.
Memory and storage
32 GB of memory suits gaming and most creative work today; 64 GB earns its price when you edit long timelines or keep many applications open. Start with one fast NVMe drive for Windows and your games, and add a second when you need it.
Power and cooling
Choose a power supply with headroom for the card you have and the one you may want next. Air and sealed liquid coolers never need refilling. A custom loop buys a quieter, cleaner machine and the look of the build itself; it asks for a fluid change from time to time.
Where to save
If the budget is tight, keep the graphics card and save on storage and memory first. Both are the easiest parts to add later, and a second drive or another pair of memory sticks fits in any of the three chassis. Spend on the power supply and cooling only as far as the card asks for.
Then the chassis
Choose the Corsa if you want more of the budget in the graphics card, the Velox for an airflow tower with vented aluminum panels, and the Aventum X if you want a custom loop built into the chassis and have the desk space for a tower over 28 inches tall.