I have used PC3200 Memory in a PC2100 fastest capable motherboard.

I have used PC2700 in PC1600.

I have also used PC133 in a PC100 ONLY motherboard... but

I have also had failures in all categories as well.

It realy depends on the Design and chipset. You might also want to update the BIOS of the motherboard while you are at it. The newer BIOS might have better support for memory types.