As far as I know there is no such thing as a MacPro Tower with only 3gigs of RAM. Where did you get this tower? And how old is it?
Anyways if you really a have a MacPro Tower and are doing video editing, first thing is you need much more then 3gigs of RAM installed. Here's some tips on what you need to do and know.
1. Memory needs to be installed a certain way because the dual processor setups. You need to install two memory sticks of the same size, one in one memory slot and the other in corresponding slot, for example
Slot 1&5 / 2&6 / 3&7 and so on. Here is a link with videos on how to install memory in all different types of macpros [
eshop.macsales.com].
2. You should have a minimum of 6-8gigs of RAM. OSX needs 2gigs on its own just to run smoothly. Yes FCP at this time only uses 4gigs of ram even if you had 64gigs installed. But as you open more apps at the same time each of these is going to use some of that ram therefor taking away from what ever else is open.
3. Now for FCP. You can't just load any type of video clip in a timeline even it is .mov. .mov is whats called a container. Even though its a Quicktime Movie. Whats inside can vary, for example the codec used to display the video. Some codecs are uncompressed (prores 422, 4444, animation) witch are used for editing and some are not (h.264, mp4) which would be used for showing on the web or sending over email, etc...
And there is also aspect ratio's, make sure its a proper 4:3 / 16x9 size SD or HD.
4. For clips not showing up. Can you open them in the quicktime player, if so they are working you just need to reconvert the videos to QT format that FCP understands. Also do not use AVI's this is more of a PC format.
Hope some of this helps.