Kitguru article on new 12 pin cable
After watching the WAN show this week I was looking if my PSU would work for the new cards. Found this article with a confirmation that Seasonic already has produced the adapter we will need for the new cards.
This should bring us the comparability we need from existing modular power supplies given you have enough power behind it. If the cable can carry 600w, than you should probably check the actual power draw at load to see if your current PSU can handle the power load. My 750W PSU would possibly be fine for lower end rtx3000 but may not be able to make it for the higher end. But I dont think I would like to even run such a high power card, given the total cost to run the card with more electricity. This may be a situation you find yourself in too, so be sure to make sure your new card is the one for you and your needs.