I've got a dual opteron 242 setup, and gimp works beautifully. Note,
however, that i'm running gentoo, so Gimp has been compiled from
scratch with a 64-bit gcc.
As for improvements over 32-bit, I can't really say, my best
comparison is comparing a 2x1.6gHz 64bit machine to a 1x0.9gHz 32 bit
machine, so no real basis for comparison.
Also, I don't know if any of the filters in GIMP have been optimized
with hand-written assembly, or if they are straight c/c++/whatever.
If they're assembly, they may actually run slower as-is on amd64 than
x86 (this has been the case with several media (de|en)coders), but
that will change when someone writes an amd64 assembly version.
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