To be honest the PSU can be a critical component, best not to scrimp on power or cost as nearly everything in the PC casing will rely on it. A cheap PSU can cause a lot of problems if it fails and can take sensitive parts with it.
As for choosing ram you can try visiting crucial.com and using the site to match your memory type to the motherboards you are looking at.
The main thing to do with your first PC build and any subsequent builds is to take your time, read the manuals, ask questions and enjoy it
It can be fun and will certainly allow you to learn a lot more about the workings of a PC.