No additional ROM images are needed. All you need is whatever 3rd party apps you want for the virtual device. For me that's HandyShopper (and once upon a time, Pocket Quicken).

Don't let the size fool you. This is ~10-year-old software emulating what was then ~10-year-old hardware. The original Palm had 512k of memory . . . total. And that includes storage as well: OS and programs were executed in place, so storage and memory were one and the same. (None of this new-fangled "Harvard Architecture".) I think the garnet devices eventually grew to 16 MB -- my Visor Prism was 8MB.

I used the GVM on my N900 all the time and I never noticed any performance problems. I didn't put the N8x0 version through nearly as extensive testing, but I think you'll find the performance meets or exceeds the expectations of users of the original hardware.