Hi Graham,
Personally, I'd push for a newer machine to work effectively. Your machine is marginal for running a complex program like ArchiCAD. AC13 works best with multicore processors, a beefy graphics card, and 4gb+ of RAM. I just don't think it would be economical to upgrade your existing machine.
If it's any help, these are the minimum requirements
here. It says the minimum RAM for running AC13 is 2gb, so it is going to struggle with only 1.5.
Regarding the processor running flat out in order to generate the sections and elevs - this is fine! But you will find that a multicore processor will do it several times faster compared to the single core on your machine.