I don't have any significant experience with Leopard other than on a Mac Pro, but I've had full FCS2 installs running on 2.5GHz G5 dualies (Tiger) since it was released and have both FCS2 and FCS3 running on Snow Leopard flawlessly.
Apple says you need an Intel processor to run FCS3, but the Get Info for FCP7 says the code is universal and therefore it may run on a G5. Irrespective of that, DVDSP4 should run fine on your G5.
As for what you can "get away with" on a MacBook, you may find that the lack of firewire on recent MacBooks a problem. Additionally, I'm not sure about the video support. The MacBook Pro is a better choice, but more expensive. You may want to look into a used Mac Pro as an alternative.