Palit is a good brand from what I've heard! And yeah I'm not as familiar with it but googling the same thing regarding power limiting/undervolting the 4090 will bring up similar results about optimizing efficiency for it. You can likely get it not exceed 350W under load and likely even get a performance boost, not even regression. The 4090 is incredibly efficient given its performance, it also draws way less power when idling than the 3090 or 3090Ti. Had no idea the price gap was that small haha otherwise I would've recommended the 4090 straight away especially given the price in energy increase you've more than likely experienced

I'd say in that case just go new 4090 unless you can get a used 3090 for half that or a used 3090Ti for a little over half. I'm surprised that the difference is that small, I guess I'm accustomed to the used market in the US which I imagine is quite a bit larger

Although since you are in Europe, what does pricing look like roughly for used 3090s/3090Tis versus new 4090s?

Yes that's likely better actually since its a much newer card (less likely to have been mined on) just make sure you look into tuning it for efficiency as the card is designed to run at 450W (which is insane). I can't direct you to any guides for the 3090Ti specifically but I'd just google 3090Ti undervolt. The 3090 should probably also be undervolted too, you really don't need these cards to be hitting their power limits of 450W and 350W respectively, tuning them to a more reasonable 325-350W and 280-300W makes way more sense.

No. I'd get a used 3090. Save the rest of your money for when you have more experience and a better grasp of the kinds of problems you'd like to solve and the hardware needed to run the corresponding models. Then you'll realize either that your hardware is sufficient as is (with a 3090), a 4090 would actually benefit you, a card with 48GB of VRAM is essential (ie. You need either an Ada Titan if it comes out or 2x 3090s), or it's way too expensive to run on consumer hardware and just use cloud GPU instances with A100s or H100s instead. But the 3090 will be a great card for now, and a used one in great condition (sometimes even open box or with an active warranty) can be found easily on the hardwareswap subreddit for $800 or even less.