My brother's got 1GiB of VRAM on Vista and he still has this problem at roughly the same level that I have with 256MiB on Linux.
However, I have noticed that Firefox is a lot more sensitive than other browsers to the optimizations for composited desktops on Linux. Konqueror, Opera, and friends work like a charm no matter how video is configured. Firefox's normal operation was sluggish enough to drive me nuts (as its normal state of being) until I finally combined Firefox 3, KWin 4.x compositing, and the tweaks to the nVidia drivers to make compositing work acceptably fast.
My brother's got 1GiB of VRAM on Vista and he still has this problem at roughly the same level that I have with 256MiB on Linux.
However, I have noticed that Firefox is a lot more sensitive than other browsers to the optimizations for composited desktops on Linux. Konqueror, Opera, and friends work like a charm no matter how video is configured. Firefox's normal operation was sluggish enough to drive me nuts (as its normal state of being) until I finally combined Firefox 3, KWin 4.x compositing, and the tweaks to the nVidia drivers to make compositing work acceptably fast.