I use Logic Express - I bought version 8 for about £14, then upgraded it to version 9. It's a universal binary that also runs on Intel machines, although off the top of my head it stops working from El Capitan onwards. I assume that some of the AU units are 32-bit. I don't know. The built-in sampler and sample library is particular good. Logic is relatively "boxed-in" in the sense that it wants to be your one and only music production studio, but the components are pretty good and the interface is simple.
Here's a "video" - it's just a still frame - of some music I made with my old 667mhz TiBook, which at the time only had 256mb of memory. Bear in mind that most of the music is pieced together with samples, which are processor-light, and external MIDI sequences:
And here's a short clip of my 1ghz TiBook running LE9 with internal instruments only. Even with 1ghz it'll only run a single instrument with effects, so it's less useful as a sequencer than as a standalone musical instrument:
On my G5 it'll run a decent load of instruments and effects, with the benefit of a larger hard drive and a PCI audio interface. On my 2.4ghz MacBook Pro it's better still. I have a hard drive with Snow Leopard dedicated solely to it. Albeton is more geared to live performance and I've always thought that it was the one killer app for stylus-operated tablet laptops - it would be great on something like an iPad Pro.
I also have a Roland UM-1 MIDI interface, which feels overpriced, but works effectively. It has no trouble sending and receiving SysEx data to my MicroKorg whereas the old EMU MIDI interface I had didn't work at all.