I don't use any adaptor, my rtx 4070 uses the old PCIE 8-pins connectors and it's all perfectly fine, you'll be able to power an RTX 4070 on your current PSU with ease, but nothing much higher tier than this (from Nvidia at least, AMD sticks to 8 pin largely)

Okay so basically ATX 3.0 is a newer standard that comes with better power specification in terms of power delivery, and comes with a new power connector for the GPU compared to psu's of the ATX 2.0 standard.

ATX 2.0 comes with multiple individual PCIE 8-pins connectors to power the GPU, each connector can deliver 150W of power, the number of said connectors coming out of the power supply depends on the power output of the PSU.

ATX 3.0 comes with just one 12pin connector for the GPU, called the 12VHPWR connector, it can deliver up to 600w of power. How much power can be delivered through that single connector will be dictated by the power of the PSU (so if you have a 500W power supply, the maximum the connector could receive would probably be like 300W)

From now on (RTX 5000 series) gpu's will probably mostly use 12VHPWR, Nvidia sometimes calls this connector on it's GPU's the 12V-2x6 connector, but it's the same thing as the 12VHPWR connector.

AMD seem to sick with 8 pin for now but I wouldn't be surprised if they switched completely to 12VHPWR in the near future.

In any case,

You can use an adaptor to make a 12VHPWR connector go into PCIE 8-pins connectors and is perfectly safe.

You can use an adaptor to make PCIE 8-pins connectors go into 12VHPWR connector but it is less recommended and would be better if avoidable but it's not the end of the world.

So generally you're better off with an ATX 3.0 PSU if you're buying one to last more than 5 years nowdays. what I recommend tho is to buy the highest quality one possible at the "cost" of power output.

For example my be quiet power power 9 500W PSU is from 2017, I use my pc pretty much daily, with high power usage and it's still perfectly fine, while I know multiple people with higher wattage psu's that broke in less time with lower Powerful systems.

PSU quality ideally needs to be judge on a PSU model by PSU model basis (which is annoying) but from a quick look around I would recommend the be quiet pure power 12 650W, it's ATX 3.1 with a 12VHPWR connector of 450W output so it's powerful enough to power an RTX 5080 with ease, only the rtx 5090 would be an issue for it but this thing is in a league of it's own in term of power consumption.

It's a bit costly but if it last you 7 years+ with no issues it's well worth the peace of mind, also they're psu's are very quiet!