A good upgradable pc to start with is $800-$850 $500 is technically doable but right around $800 you'll get a setup that's Better for upgrading over time. And tip from an IT worker whose built pc for years. Don't shop by brand for the parts shop what's parts are best specifically at YOUR budget. Nvidia having the 4090 be the best graphics card out means nothing that's not in your budget.

You only buy a decent GPU / CPU with 500$

Is that a joke? You can get half of one component for that price

step 1: get a friend who is knowledgeable about computer parts step 2: go to facebook marketplace step 3: search for "gaming computer" step 4: call friend step 5: have friend analyze offers step 6: find a good ass deal/buy computer step 7: clean the computer/set it up step 8: enjoy! you want to search something simple that way you can find all the offers from people who have no idea what they're talking about. i saw one the other day that could fetch $800 for $200. good luck, have fun!

The Radeon RX 7800XT is $500 itself