Thanks Sven, sorry for the delayed reply, got busy with some 3D printing work.
The friend who loaned the machine we're using now should be providing our original pc soon with 64-bit Win7, so hope that will take care of it. Don't recall its or the loaner's graphics card specs.
In VMWare Fusion 6 on my Mac, I have Win7 64-bit and only 2GB memory assigned and it fused at 1000/closed in about 90 seconds, saving the result as obj stalled at about 80% after crunching for about 2 minutes; looked again after another 5 min or so and it was done. Saving stl took only about 45 seconds. Will bump up the memory there to 4GB or more and see how it works. The fuse result looked good on screen but then I tried to open it with other fuses at lower settings and the one done on the mac was visual gibberish, white, brown, black triangles and other shapes, not the quite-nice-looking earlier version.
I'm not a software developer so I'm not sure how hard it would be to have DAVID present a warning or error message when it encounters memory fragmentation, with an option to cancel, clear the memory, etc., but as a user, that would be mighty fine to see that instead of not knowing, and seeing the 'Fusing Scans...' with no red bar or any other indication of activity or error.
Anyway, thanks, now I have a better feel for what the issue may be and if DAVID gets unresponsive, I'll think about quitting it and re-launching.