In tests I did years ago, Call of Duty was the first Mac game that showed any advantage of 256MB of VRAM vs 128MB. COD only supports 10.2.x+, so, sadly no OS 9 version.
Some OS 9 games may show a little benefit from 256MB if we are talking insanely high game resolutions, tho you'll likely be CPU/Memory and bus bandwidth bound.
OS9 didn't support FSAA, except with 3DFX hardware, and cranking up FSAA would be where we would expect to see the most gains from more VRAM, however, the R9200 GPU clock would also become a limiting factor.
So, the short answer is, no, you're not really going to see any real world benefit to 256MB of VRAM vs 128MB under OS 9, but it will look cool in the System Profiler.
