It is a real challenge to use FCPX on an old computer. You need all the RAM you can get. The other problem is the video cards can’t handle the native H.264. I gave up on my iMac7,1 and went to a new 27″ iMac with 8 GB of RAM. Life is easier now.
Not sure on the 422 question though. I would try a very short part of your project to see if it is feasible, before getting to far in. My first project on my old iMac was nerve racking because of all the crashes.
What about getting a 500 GB portable drive to help with the project?