I am a student and we use FCP 4 on Powermacs with 60Gig hardrives at school. Ram is what is important, at least 512. At home i've got one of my powermac quicksilvers running FCP4 on a single 933 and a dual 1.44. I find with maxed out ram(1.5Gig) on a single 933, FCP 4 can be a bit tempermental but on the dual, it flys. I suggest using duals.
I've tried using FC Express on my single and it worked fine. The only thing is, once my files started getting realy big, FC started running slower. It could be because of the single processor and the out dated video card though. I hope this helped. 😀