Well, here's what I found:
latest drivers, themes disabled, no audio, XP pro sp1
DDR400:
sw2005.Result.score = 1.46 sw2005.Result.graphics = 1.33 sw2005.Result.cpu = 1.62 sw2005.Result.io = 1.49
DDR333:
sw2005.Result.score = 1.42 sw2005.Result.graphics = 1.29 sw2005.Result.cpu = 1.59 sw2005.Result.io = 1.43
DDR266:
sw2005.Result.score = 1.41 sw2005.Result.graphics = 1.28 sw2005.Result.cpu = 1.59 sw2005.Result.io = 1.42
DDR200:
sw2005.Result.score = 1.37 sw2005.Result.graphics = 1.23 sw2005.Result.cpu = 1.56 sw2005.Result.io = 1.37
It looks like upgrading memory speed is worth just a few percentage points. I should do some more tests to see if my CAS 2.5 is worth the bother....