First you ask about the case and now you're asking about the motherboard and PSU? Which one is it?
If it's the case, you either know the make/model of your case and then looking up published numbers in regards to how long of a GPU a case will fit OR you break out the ruler and measure out how much room you have.
If it's the motherboard, as long as the motherboard has a PCIe x16 slot then pretty much any GPU made in the last 5 years will fit.
If it's the PSU you can refer to this guide - http://www.realhardtechx.com/index_archivos/Page362.htm. Look up the GPU you want and figure out from the link the recommended PSU wattage.