I have been using FCP 3 (previous version) on a tibook 400 with 512 mb RAM and my 14" ibook is quite a bit faster than that. If you max out the RAM I would think FC Express would work just fine. I'm planning to run FCP. I'll let you know how it works once I get it loaded.
You might take a look at the specs for FC vs. iMovie. I've noticed that Apple's "Pro" products often times have lower minnimum requirements (RAM, Processor power etc.) than their iApp counterparts.
