Ok I'm back with a question, will an rtx
fit in the case even if I change the motherboard?
You would have to get measurements; there's no standard case or GPU size. You would need to get a tape measure, open up your PC, and measure the clearance in the area the GPU would go.
As for PSU -- sorry I missed the follow-up -- it would depend on the GPU you want to go with, your other hardware, your country, and your budget. But don't just buy a GPU and then
choose a PSU; people sometimes spend all their money on a GPU and then don't have the money for the proper PSU.
The only good news about that frightening PSU you currently have in the system is that at least you know your motherboard can take a standard ATX PSU rather than requiring a proprietary Dell PSU, which can really complicate issues (and sometimes results in needing a new motherboard and case).