Thanks for the reply Eric, you've confirmed my suspicions. I was talking about my 9 year old powerbook G4 & G5 Powermac hence the expensive upgrade path for Intel Macs and Binab, (don't even mention graphics software)
The good news is that I must have improved as a musician as I now play in a couple of groups with real people - when we started we did use Binab backing tracks.
All improvements entirely due to Band in a Box which in my view is a superb learning tool making learning really enjoyable. Perceived limitations in the program are more likely to be because you've improved as a musician!
I think for the moment I will nurse my existing set up which has served me so well - now, back to Nica's Dream.