If you're going to try to upgrade the power supply, you should research what options were available from the factory for whatever model you are thinking of upgrading. For example: Dell sells the XPS 8940 with either a 360W unit, or a 500W unit, depending on how it is optioned out. They have done the same in the past with other models. These power supplies are not generic ATX units, and cannot be interchanged with them. They are designed for the specific PC they are installed in. So while you can't swap in an ATX PSU into these machines, you can upgrade from the 360W unit to the 500W. I have seen a Youtuber do this when he wanted to swap the video card that the machine came with for a more powerful one and the 360W wasn't enough.
A point to keep in mind, though. Just because a prebuilt computer came with a certain video card as an option, doesn't mean that all retail cards using the same GPU will work. After doing some research, I have discovered that Dell contracted with MSI for the cards in the XPS 8940, G5 5000, and probably other models as well. The XPS 8940 and G5 5000 list the 3060 Ti and 3070 as upgrade options, but these are NOT the same as the cards that you buy at retail. They are lower power versions that work with the 500W PSU that Dell uses. I haven't researched the 3060Ti yet, but I did research the 3070 and found that MSI makes a version that only requires a single 8 pin and a single 6 pin to power it, where most other retail 3070 cards require either nVidia's 12 pin connector or dual 8 pin connectors. It's called Twin Fan, but I haven't seen it for sale anywhere in North America yet, so it may be regional to Asia, Europe or some other part of the world, or it hasn't officially been launched yet. It is on their website.
So keep in mind the power demands of the card you are thinking of installing to make sure it stays below what the power supply can handle and that it has the same types of connectors that the power supply has. It shouldn't be an issue with a 1650, though, as long as it isn't a 1650 Super, as there is no external power required for these cards.