Just because your case is large enough to fit an ATX motherboard doesn't mean one will actually fit. Have you checked that the mounting holes are in the right place? Also, if the power supply is the one that came with the case it's probably past its best-before date and should be replaced as well.
To answer your question, as long as you stick to the main four manufactuers (ASUS, ASRock, Gigabyte and MSI) it shouldn't matter. Even their cheapest boards have decent quality components. RAM shouldn't be an issue either since I don't even think there's a single modern motherboard that doesn't support at least 16GB RAM.
Just to make things easy, post the current model of computer you have, the model of the graphics card and power supply and your maximum budget and I could put together a build for you.