I once had a 1.42 Ghz Mini G4 with Logic Express 7 on it, and a friend of mine is now using it. He's also playing in a band, but I'm not sure if he uses the Mini there. The G4 definitely was underpowered for Logic Express 7 when you had more than a fistful of tracks.
I have Logic Express 8 on an iMac Core Duo 2 GHz, but I am not using it for hard disc recordings. (I'm not really doing anything with it, at the moment - photography is consuming most of my time right now, and learning Final Cut Pro). But what I can tell you is that the iMac handles the demos that come with Logic 8 very well; both the ones with software instruments and the ones with audio tracks only.
I have Logic Express 8 on an iMac Core Duo 2 GHz, but I am not using it for hard disc recordings. (I'm not really doing anything with it, at the moment - photography is consuming most of my time right now, and learning Final Cut Pro). But what I can tell you is that the iMac handles the demos that come with Logic 8 very well; both the ones with software instruments and the ones with audio tracks only.