RAM is capped at about 3 GB. I've seen it use a little less than 3 cores at the most, for Final Cut Pro. Motion, I haven't done that much testing with. Color, either, but I think they can both use more than Final Cut can.
Compressor can use as many cores as you have, and I regularly use all 8 of mine to shorten encoding times.
I depends on the Final Cut app. Final Cut Pro rarely uses more than 3GB of RAM, as TheStrudel has pointed out (it's still a 32-bit app). Some rendering tasks within FCP will bump CPU utilization across all CPUs in the mid 70s on my 8-core 2.8, but that's pretty rare.
Compressor will use all the CPU resources you allow it to use in QMaster.
Color and Motion rely heavily on GPU, but don't scale CPU usage very well. Incidentally, I really wish that Color was more multi-CPU aware for render-outs. It desperately needs that.
Frankly, this is a lot of the reason why I haven't really upgraded past 8GB of RAM in my machine. FCS simply doesn't use it. But the new After Effects CS5 is really starting to temp me to. That app LOVES RAM.