I'm using FCP on a 2.2 Ghz MBP. I'm working in ProRes 1920x1080, which I believe is easer for real time playback than HDV. No problems, except I get the red render bar quicker than someone with a faster computer, so RT playback is limited.
I'm pretty tired of the render bar when just adding a few filters/effects, so I've been reading about Adobe Premiere Pro CS4, which does more RT. I've heard that it needs 8 GB of RAM to be really stable, but another person said he was having no problems with 4 GB.
Check out the Adobe Production Premium before you buy FCP. You'll also get free tech support, at least until 3 months after CS5 comes out.
The reason I'm sticking with FCP is primarily because I'm editing with AVCHD, which FCP can transcode to ProRes. PPro does AVCHD natively, which I have read will not work well without a quad to eight core machine.
If you do go with FCP, I highly recommend the Larry Jordan tutorials on lynda.com.
(I'm no pro, so take this for what it's worth).😀