Many memory chips are multi speed, and some are single speed.  That is, a pc3200 chip may operate as pc2100, pc2700 or pc3200, or just as pc 3200, or as pc3200 and pc2100.  Generally it will run at the highest speed the mb can handle.  Install a freeware product that can check your memory (sisoft sandra lite is one), and/or check your bios when you boot to see what the pc shows is in use.  the mb also has limitations, might have a max of 512 or 1gb in each slot, and may be running you pc3200 chip as pc2100.  Win xp 32 bit doesn't really support more than 3 mb ram.      
Paul Hachmeyer

From: "Tom Waddell" Using PC3200 in a PC2100 MB

Have an older MSI MB. Mfg says to use PC2100 for best results, otherwise system could become unstable. Currently have 512MB (2x256)
PC3200 in MB and it works fine. Want to replace memory and up-grade to 2GB. How much of a problem am I likely to have using the
PC3200 memory? Does problem become more noticeable with increased memory?

    
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