XP peaks with 512 mb memory. Anything more is simply overkill, in my opinion.
The other issue is your bus speed. If your motherboard will not run the pc3200 at the full 400mhz, then there is no advantage to having the faster memory. While it may work just fine, it will only run as fast as the motherboard bus will allow.
I had 512 of pc2100 on my xp pro system running a athlon xp 2400 and abit n7f motherboard. For about $30.00 more I got cosair 1 gig pc3200 for what it would hae cost for 1 512 of crucial pc2100. Now why does not my system show an improvement in preformance due to the faster memory and higher bus speed.