You should be able to run 2x512MB + 2x256MB in dual channel.
However, because you are running 4 modules, the system may clock them down to DDR-333 for stability concerns.
In most cases, 2x512MB is a better memory setup under WinXP.
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<font color=green>"My Memory is not working today"
<font color=blue>AIM SoCaliCrowley