About four years ago I installed Ardour 0.9x on a tiny 256MB 533MHz PIII for a friend, which is reasonably close to the specs of our dear N900. I believe it had a real PCI audio card in its only PCI slot

Back then, Ardour wasn't functionally usable without the jackd realtime audio daemon, and the best latency I could get in jackd was 128ms for mono recording, and 256ms for stereo. Basically, recording alongside an existing track required timeshifting each track manually due to the audio lag. And forget about non-destructive plugins! In other words, I guarantee you Ardour on N900 is more hassle than it's worth.

Fillmore is a Maemo app that appears to fill the same role as Ardour does, but with a slick phone-optimized interface. Unsurprisingly, the same performance caveats apply -- tablet mode and overclocking seem essential to its usefulness.