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In detail:

CS4 starts with 62MB of ram, 0-2% CPU used. I run a pass of greyc(on a 600dpi B5 scan) during which 700MB of ram, 98-100% CPU is used. After it is done, the ram usage goes to 300-320MB and the CPU to 2-6%. Each pass adds on a little more after it finishes, till Photoshop crashes or Greyc throws bad allocation.

Closing the image, Edit > Purge > All, have no effect. Also same thing with CS3