High Octane Hardware For GIMP Use?

green pizza asks: "My research group will soon be purchasing several workstations for telescope image analysis. We are currently planning on going with dual Pentium III systems from VA running Redhat 7. We have a good deal of custom filters and scripts that will be churning away in The Gimp and would like the best performance possible. Is our current choice the best one? Should we consider moving up to a Xeon system or perhaps a high-end, multiple processor Sun Blade 1000, SGI Onyx 3000, or Alpha?"