No, you don't need a fan, although the board may overheat if you keep it on longer than a few minutes without a fan. You can power on a system that is as minimal as yours, although you're really not going to be able to learn a sufficient amount about it and whether it's working or not without the screen assembly. You can hook up an external display if you have an adapter. Without some means of seeing video, though, and no RAM, you're really just looking to hear some audio beeps (assuming your speakers work). But a board that beeps is still far from being confirmed "good", so I'd get a hold of a screen assembly so you can do some real testing. On second thought the external video is not such a great idea because lots of this model have no backlight (a board issue), which you wouldn't notice on an external display.