Making a game, some tips on making UI for it

So I'm in the process of writing a document that details every item in my rpg that I could possibly think of. After that I will either make or find appropriate images. Has anyone here done UI art before? What are some tips you guys can give? I have Photoshop, will that be enough? Should I get another program and do vector art so that it scales well?

That's all I wanted to ask...back to writing.